Astronomische Nachrichten

Volume 319 1998 Number 4

Astron. Nachr. 319 (1998) 4, 201-214

Identification of new low-mass members of the Alpha Persei open cluster by ROSAT. I.

C.F. PROSSER: Cambridge, Massachusetts, USA

Harvard-Smithsonian Center for Astrophysics

S. RANDICH~ Garching, Germany

European Southern Observatory

Received 1998 April 30; accepted 1998 July 1

Following the work of Randich et al. (1996) involving a ROSAT raster scan survey of the a Persei open cluster, we present here the results of a photometric/spectroscopic program examining the possible optical counterparts to a group of 73 X-ray sources in the raster survey which were not matched to catalogued stars. Of the 73 sources investigated, N 40 have an optical counterpart with photometry acceptable for cluster membership and N 20 of these also have radial velocities consistent with membership. We discuss the X-ray properties of these potential new members and why they may not have been identified in earlier membership surveys of this cluster.

Key words: open clusters and associations: individual (a Persei, Melotte 20) - stars: late-type - stars: coronae - stars: activit,y - X-rays: stars

1. Introduct ion

The Q Persei cluster has been surveyed by an extensive series of ROSAT PSPC pointings, forming a raster scan covering about 10 sq. deg. About 160 X-ray sources were detected by Randich et al. (1996); 89 of these were identified with known objects in the a Per region, allowing Randich et al. to study the X-ray properties of solar- type and low-mass dwarfs in a Per. On the other hand, no known optical counterparts were found for the remaining - 73 sources. In this paper we present a study of these 73 remaining unidentified X-ray sources. Extensive use of both photometric and spectroscopic data has been employed in identifying possible optical counterparts to the X-ray sources, many of which, as will be seen, represent newly identified cluster members or candidate members which were not identified in previous membership surveys of this cluster.

Our paper is organised as follows. A review of the X-ray data is given in Sect. 2, while the optical data is discussed in Sect. 3. A discussion regarding the results is given in Sect. 4 and a summary in Sect. 5.

2. X-ray observations

As a complete discussion of the X-ray data and source detection is given in Randich et al. (1996, their Sect. 3), it need not be repeated here except for a brief summary. The raster scan consists of 34 pointings with exposure times on the order of 1-2 ksec, covering 10 sq. deg. over the central cluster region. An effective sensitivity of Lx 1028.8-29 ergs s-l was achieved in the central portion of the raster scan where several pointings overlap.

As described in Randich et al. (1996), source detection was performed in both broad (0.1-2. keV) and hard (0.4-2. l e v ) bands by means of the Maximum Likelihood Algorithm (ML; Cruddace et al. 1988). Sources with

*National Optical Astronomy Observatories, Thcson, Arizona USA tosservatorio Astronomico di Arcetri, Firenze, Italy

14 Astron. Nachr. 319 (1998) 4

202 Astron. Nachr. 319 (1998) 4

ML 2 7 in hardband were accepted as real, although a few sources were shown to be bad detections after visual inspection of the data and were subsequently dropped. Out of the - 160 detections from the ML algorithm, 73 sources were not associated with catalogued or known stars in the region. As noted in the introduction, the 89 sources associated with catalogued stars have been discussed by Randich et al. (1996). The 73 X-ray sources in this study are given in Table 1 where they are listed in increasing right. ascension. Included in Table 1 are a running APX ('Alpha Per X-ray') number, the 2000 coordinates of the X-ray source, the hardband ML value, the effective exposure times, the net count rate and error in the count rate. Finally, a measure of the hardness ratio is given, where HR = (H-S)/(H+S), with H and S being the count rates from the hard and soft (0.1-0.4 keV) bands respectively.

Table 1. Raster APX X-ray Source List

4 PX (2000) ID KA DEC MLH

COlllll. Hale Exp. Hate Error HR (s) (s-1) (s-1)

APX (2000) COlllll H,ak

(s) (s-1) (3-1) ID RA DEC MLH Exp. Hate Error HR

- I 2 3 4 5 6 7 8 9 10 I I I 2 I3 14 15 I6 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

3 20 52.8 +4Y 27 53 35.1 3 21 05.0 +47 55 24 25.9 3 21 27.6 +49 56 58 9.4 3 21 53.0 +49 04 17 160.4 3 22 37.5 +49 37 56 8.8 3 22 41.5 +48 I6 08 124.9 3 23 06.3 +49 25 46 7'2.2 3 23 08.0 +48 38 05 24 1.4 3 23 15.5 +4Y 34 59 46.2 3 23 15.5 +49 22 08 18.0 3 23 23.5 +49 29 24 8.5 3 23 29.5 +47 43 I I 11.7 3 23 32.6 +4Y 07 28 12.8 3 23 34.4 +49 I3 22 42.5 3 23 48.9 +49 16 20 21.8 3 23 50.2 +49 05 31 234.9 3 24 03.0 +47 36 42 17.1 3 24 05.9 +48 31 23 21.6 3 24 11.4 +48 45 46 30.7 3 24 12.6 +49 08 59 36.6 3 24 18.9 +4Y 12 46 22.0 3 24 21.9 +49 04 43 13.4 3 24 26.1 +49 14 49 14.3 3 24 35.0 +49 43 10 144.5 3 24 35.1 +48 01 15 21.3 3 24 38.5 +49 59 42 7 I .8 3 24 42.9 +49 53 04 100.9 3 24 59.4 +48 52 55 470.8 3 2 4 5 9 . 6 + 4 8 4 8 l l 61.7 3 24 59.9 +48 37 01 16.3 3 25 11.2 +49 04 14 41.6 3 25 28.9 +48 57 50 36.8 3 25 31.1 +47 49 20 14.2 3 25 51.1 +48 39 18 20.4 3 25 52.7 +48 58 56 13.2 3 25 54.6 +49 09 01 60.6 3 26 59.9 +48 42 34 26.3

4717 0.008 0.002 +0.6 4492 0.004 0.001 -0.1 1883 0.010 0.003 +0.0 9171 0.013 0.001 -0.3 5397 0.003 0.001 +0.1 9809 0.009 0.001 +0.7 8142 0.008 0.001 +0.7

14638 0.0098 0.0009 -0.3 6688 0.007 0.001 $0.5 9021 0.0019 0.0006 +0.2 8732 0.0014 0.0006 -0.2 4582 0.004 0.001 +0.1

I226U 0.00z 0.0006 +0.5 I1608 0.0037 0.0007 +0.5 11653 0.0017 0.0005 +0.2 13152 0.014 0.001 +0.8 4990 0.004 0.001 +0.1

15766 0.0014 0.0004 +0.5 16356 0.0025 0.0005 +0.6 13158 0.0032 0.0006 +0.4 12886 0.0031 0.0007 +0.3 14255 0.0015 0.0005 +0.1 12357 0.0022 0.0006 +0.2 V267 0.023 0.002 +0.8 9658 0.0029 0.0007 +0.5 2900 0.033 0.004 +0.7 5186 0.023 0.003 +0.6

17900 0.020 0.001 >0.9 18785 0.0037 0.0006 +0.7 18985 0.0013 0.0004 +0.1 14903 0.0036 0.0007 +0.7 18173 0.0024 0.0005 +0.4 9784 0.0027 0.0008 +0.3

196080.0017 0.0005 +0.2 17515 0.0012 0.0004 +0.2 17915 0.0040 0.0006 +0.6 21824 0.0018 0.0004 +0.4

38 3 27 05.7 +49 41 46 7.6 7471 0.0026 0.0009 +0.0 39 3 27 24.4 +47 02 58 42.9 8'20 0.051 0.009 +0.5

42 3 28 11.6 +48 56 09 10.9 19126 0.0014 0.0004 +0.1

40 3 27 33.3 +47 55 14 98.6 14821 0.0067 0.0008 -0.1 41 3 27 45.3 +48 I6 05 11.8 10217 0.0015 0.0004 +0.1

43 19974 0.0126 0.0009 +0.7 44 3 28 35.6 +49 25 14 13.7 I1031 0.0017 0.0006 +0.1 45 3 28 37.2 +47 55 14 14.8 12631 0.0019 0.0006 +0.2 46 3 28 39.9 +48 46 14 9.7 19007 0.0008 0.0003 +0.0 47 3 28 43.6 +49 29 11 9.8 10058 0.0018 0.0007 +0.3 48 3 28 49.9 +48 20 14 14.7 16797 0.0010 0.0003 +0.0 49 3 28 53.1 +47 29 44 18.6 7181 0.007 0.001 +0.4 50 3 29 00.2 +49 43 54 16.1 5835 0.005 0.001 +0.2

52 3 29 05.8 +49 19 01 10.0 13391 0.0014 0.0005 +0.3 53 3 29 37.3 +48 17 34 53.1 18744 0.0024 0.0004 +0.4 54 3 29 41.4 +48 34 41 107.2 20428 0.0032 0.0005 +O.G 55 3 29 50.5 +48 06 29 8.5 15516 0.0008 0.0003 +0.0 56 3 30 13.4 +48 04 25 16.4 13057 0.0015 0.0005 +0.1 57 3 30 17.0 +47 55 23 13.3 11310 0.0016 0.0005 +0.1 58 3 30 33.0 +48 30 37 66.5 16570 0.0034 0.0006 +0.5 59 3 30 33.6 +49 27 13 109.2 8'261 0.011 0.001 +0.7 60 3 30 34.9 +48 19 35 11.0 14322 0.0011 0.0004 +0.0 61 3 30 42.3 +49 03 03 318.2 11836 0.019 0.001 +0.7

63 3 31 16.0 +48 25 14 26.0 1331 1 0.0024 0.0006 +0.3 64 3 31 35.0 +47 51 12 10.5 10420 0.0014 0.0005 +0.0 65 3 31 44.5 +48 06 20 27.8 11644 0.0021 0.0005 + 0 2

67 3 32 09.7 +49 26 13 97.8 6029 0.017 0.002 +0.1

69 3 32 40.0 +47 46 38 9.3 6420 0.0021 0.0008 +0.1 70 3 33 20.1 +47 57 10 20.4 8191 0.0031 0.0008 +0.0 71 3 33 28.5 +48 07 20 18.2 9030 0.0027 0.0008 +0.5 72 3 33 30.9 +48 02 45 28.5 9082 0.0033 0.0008 +0.3 73 3 36 53.1 +47 38 24 39.5 975 0.046 0.008 +0.5

3 28 17.3 +48 40 47 379.1

51 3 29 05.0 +48 03 55 10.8 13578 0.0010 0.0004 -0.2

62 3 31 03.5 +47 59 55 12.6 11488 0.0020 0.0006 +0.0

66 3 32 09.1 +48 04 26 93.1 11421 0.0058 0.0009 +0.6

68 3 32 13.5 +49 14 20 273.4 7609 0.029 0.002 -0.3

3. Optical observations

In order to properly identify the likely optical counterparts to the unidentified X-ray sources, and to search for possible new cluster candidates, it was necessary to obtain both photometric and spectroscopic observations for those stars/objects near the X-ray positions. The low galactic latitude of Alpha Per (b N -5") means that several field stars may be found close to any given X-ray source. While in many cases a single star/object could be identified as the most likely counterpart to a X-ray source, in some situations the identification of the optical counterpart was not unambiguous.

3.1. Photometry

BVI CCD photometry was obtained during 1994 November/December and 1995 October using a 20482 CCD on the 1.2m telescope of Whipple Observatory at Mt Hopkins, Arizona. For each night's observations, a set of extinction

C.F. Prosser and S. Randich: New low-mass members of the a Persei cluster. I.

t ' l e b d 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

- 8,"o - Alpha Persei cluster APX optical candidates: + -

-

- - -

0 + 0 0

>. 14 - a +o .++ -

.*. O&ff,+ +o -

~ ~ ~ o " o o o -

0

- .. 3 o o o 0 ?*.. . 0 ob + -

16 - - . I . . 0 . J%:J;oo -

- . *:. '.' . 0 * % o o o -

- - - % o + o 0

- . 0 . .* $ 5 .

- - +ow:+o . ... . 9.- #

0. .. 18 -

't .,'* , I 1 1 1 I I I I 1 1 1 4 1 1 1 1 1 1 1 1 1 1 1 1 I I I I

1 1.5 2 2.5 3 0 .5

203

Fig. 1: V VS. V-IK diagram illus-

sequence of the a Per cluster (open circles) and the 'APX' optical coun- terparts located near x-ray sources. Those APX stars considered as can- didate members or undecided on the basis of photometry are indicated by crosses, while nonmembers on the ba- sis of photometry are indicated by filled

trating the lower main and pre-main

3.5

stars were monitored, along with standard stars from Landolt (1992), Joner & Taylor (1990), Schild (1983) and some a Per members with photometry from Stauffer et al. (1985, 1989) and Prosser (1992). Each night's instrumental magnitudes were transformed onto a standard BV (Johnson) and I (Kron) system. Standards with Cousins-system V-I colors were transformed to the Kron V-I system using the transformation equations of Bessell & Weis (1987). The standards were also used in determining the nightly aperture corrections applied to the photometry.

For each X-ray source, typically several stars within 60 arcsec of the X-ray position were measured for photome- try. In choosing which objects to measure we took into consideration criteria such as location of the object relative to the X-ray position, the brightness and/or color of the object, and characteristics of the X-ray source such as the strength of the X-ray emission and its off-axis location or possible blending with other sources. In Fig. 1 we show a V VS. V-IK diagram containing these stars, termed 'APX' stars for 'Alpha Per X-ray' stars, located near an X-ray source. For comparison, the lower main and pre-main sequence of the a Per cluster is shown. While many of the APX candidates appear to be background stars lying below the cluster sequence, there are several APX stars which are found to lie along the cluster sequence, particularly in the 12 < V < 14 and V > 16 regimes. More specifically, out of 73 sources, 40 cases have stars that appear to have photometry acceptable for cluster membership (i.e. generally within N 1 mag above and - 0.3 mags below the cluster sequence, allowing for such effects as binarity, photometric errors, variability, and spatial extension of the cluster) and were nominally identified as the most likely counterpart to the X-ray source and flagged as potential new members. In the remaining 33 cases no stars with photometry consistent with membership were present close to the X-ray source, suggesting that either the X-ray emission comes from a random field star, that it comes from an extragalactic source (particularly in those cases with a large X-ray to optical flux ratio) or possibly that, in the case of very weak sources, that the 'X-ray source' is really a spurious detection and does not exist.

While photometry consistent with membership is an obvious condition for a star to be a member, it is not a sufficient condition; in other words, even photometric candidates might actually be field stars which happen to be located on the cluster sequence. Therefore, to better assess their membership status, it was necessary to obtain spectroscopic observations for these photometry candidates.

3.2. Spectroscopy

3.2.1. Ha spectroscopy:

Low-dispersion spectra at H a were obtained for almost all the fainter APX photometric candidates using the Multiple Mirror Telescope (MMT) Blue Channel spectrograph, using grism 832 in 1st order with a larcsec slit to provide 1.9A resolution and a dispersion of about 0.715 &pix. The typical S /N in the region of Ha was - 30. The Ha spectra were used 1) to assess the presence/absence of Ha emission which may be expected to be seen in late-type cluster members and 2) to obtain a radial velocity measurement based on any strong H a emission feature observed. The Ha emission field stars GL285 and GL873 were observed each night as radial velocity standards from the work of Marcy & Benitz (1989). After flatfielding, the spectra were wavelength calibrated using the onedspec package in IRAF. Wavelength calibration was performed using a pair of thorium-argon exposures before and after each stellar observation. Cross-correlation for radial velocity determinations was performed using the xcsao routine

I4*

204 Astron. Nachr. 319 (1998) 4

in the rvsao package in IRAF. An average radial velocity for the target star was obtained from the two velocities as derived with reference to GL285 and GL873, which generally agreed to within f l - 2 km s-l. Due to the influences of the Ha emission profile and strength of emission, an error on the order of at least f 5 km s-l may be expected for radial velocities as derived from these low-dispersion spectra. A typical detection limit for the measurement of the Ha equivalent width is of order 0.5 A. 3.2:2. Echelle spectroscopy:

High dispersion spectra were obtained for the brighter APX candidates using the echelle spectrograph and Reticon detector at the MMT during 1995 December/l996 January. This spectrograph provides one echelle wavelength order per exposure; the a Per spectra were centered at N 6450A, with a usable wavelength range from 6430 to 648Ow for radial velocity analysis. The spectral resolution is - 0.15w, with typically S/N - 20. To remove the influence of spectrograph flexure, each observation was bracketed in time by a thorium-argon comparison lamp exposure. As with the low-dispersion spectra, the spectra were wavelength calibrated using the onedspec package in IRAF using the average of the calibrations for the thorium-argon exposures taken before and after each observation. Up to six radial velocity standards from the list of Latham & Stefanik (1991) were observed each night and an average velocity for the target star was determined from the velocities found using each of the standard stars observed on a given night. From comparison of the radial velocities (wrad) derived for a given target star using different radial velocity standards, the accuracy of the w,,d measures is considered to be only 1 km s-l at best (for narrow-lined spectra) and is dependent on the star’s rotational velocity (w sini) . Among very rapid rotators (i.e.

sini > 30 km s-l) uncertainties of several km s-l will result. In addition to observing photometric candidates, we note that in a few cases echelle spectra were obtained for objects with photometry inconsistent with cluster membership, but which were still possible optical counterparts to a X-ray source.

To determine rotational velocities from the echelle spectra, an observation of the slowly rotating star HR996 (v sin i = 5.6 km s-l, spt=G5V, Soderblom et al. 1989) was chosen as a narrow-lined template. This template was artifically broadened in increasing amounts by convolving the template spectrum with the rotational broadening function (Gray 1976). The artifically broadened spectrum was cross correlated with the original template and the FWHM value of the cross correlation peak measured. A calibration curve between FWHM and v sin i was obtained after repeating this process over a range in w sin i. The a: Per target star spectrum could then be cross correlated with the template, a FWHM value measured, and a corresponding w sini value determined from the calibration curve. In some cases a w sini value could not be derived from the echelle spectrum either due to the very high rotation which would tend to ‘wash out’ the available spectral lines, or due to insufficient signal-to-noise in the spectrum needed to provide a strong, well-defined cross correlation curve. The accuracy of the w sin i measurements is believed to be N 2-3 km s-l for 10 < w sin i < 30, increasing to 5 km s-l or more for higher w sin i .

4. Discussion

4.1.

In Table 2 we list the available optical data for likely counterparts to the X-ray sources in Table 1. For each X-ray source we list one or more possible counterparts, corresponding optical coordinates, available BVI photometry, and the number of observations in each filter. Optical coordinates were derived from Digitized Sky Survey images using the dssfinder routine within the finder package of IRAF. For those stars considered to be members or possible members (Y/Y?/? membership status) a reddening corrected B-V, is given, along with the X-ray and bolometric luminosities and log(Lx/Lb,l) value. Spectroscopic data is listed, noting the presence/absence of any H a emission, a measure of the H a emission equivalent width and the radial velocity based on the Ha emission feature in the low-dispersion spectra. For brighter stars, radial and rotational velocity information based on the echelle observations is given. A membership flag on the basis of radial velocity and an overall or ‘final’ membership flag is then given. These membership flags are qualitative in nature and signify members/likely members (Y/Y?), undecided cases (?), and nonmembers/likely nonmembers (N/N?). In assigning membership, we have chosen what we consider to be reasonable membership determinations based on the available data and an understanding of the measurement uncertainties. Additional observations, such as to determine binarity or presence of lithium, may refine the membership estimates in Table 2. The ‘APX’ prefix is used to identify the optical counterparts, except in a few cases where an A P (Prosser 1992) or HE (Heckmann et al. 1956) star was identified as the possible optical counterpart near the X-ray position. Bolometric luminosities were obtained using the bolometric corrections (BCV) from Harris (1963) for stars with B-V, 5 1.20 (approx. spectral type K5 and earlier), while BCI values were obtained from Monet et al. (1992; eqn. 6) for later-type stars. Finding charts for these stars in Table 2 are provided in Appendix A. Information is also given in Appendix A for those X-ray sources for which a definitive optical counterpart could not be determined.

Optical counterparts to X-ray sources:

C.F. Prosser and S. Randich: New low-mass members of the cy Persei cluster. I . 205

Table 2. Optical counterparts

APX Star RA D E C V B-V V-IK vbi B-V, log log Phot H a H a H a Echelle Vrad Final (2000) Lx LX/Lbol Mem Emiss EW(A) vrad vrad vsini Mem Mem

1 APXl 2 APX2 3 APlOl 3 APX3D 4 HE497 5 APX5 6 APXG 7 APX7 8 HE553 9 APX9 10 APXlO 11 A P X l l 12 APXl2A 12 APX12B 12 APXl2C 12 APX12D" 13 APX13 14 APX14 15 APX15 16 APXlG 17 APX17A 18 APX18 19 APXl9 20 APX20 21 HE606 21 HE604 21 HE600 22 APX22A 23 APX23A 23 APX23B 24 APX24A 25 APX25A 25 APX25B 25 APX25C 26 HE615b 27 APX27A 27 APX27B 28 APX28 30 APX30A 30 APX3OB 31 APX31 32 APX32 35 APX35A 35 APX35B 37 APX37A 37 APX37B 38 APX38 39 HE749 40 APX4O 42 APX42A 42 APX42B 42 APX42C 43 APX43A 43 APX43B 43 APX43C" 44 APX44C 45 APX45A 45 APX45B 46 APX46 47 APX47 48 AP76 49 APX49A

50 APX5O 52 APX52 53 HE860 55 APX55 56 APX56A 56 APX56B 56 APX56C" 57 HE888 58 APX58 59 APX59 61 APXGl 62 APX62A 62 APX62B 63 APXG3 64 HE955 65 APX65A

49 APX49C

3 20 51.5 49 27 59 12.57 0.92 0.81 3 21 03.9 47 55 23 17.64 ... 2.98 3 21 22.1 49 57 03 13.89 1.25 1.24 3 21 21.3 49 57 41 15.61 1.59 2.63 3 21 51.6 49 04 18 5.93 0.46 ... 3 22 35.7 49 38 05 15.5B 1.48 2.20 3 22 41.5 48 16 09 13.15 1.29 1.34 3 23 05.4 49 25 59 11.94 0.81 0.67 3 23 07.9 48 38 02 8.3 . . . . . . 3 23 15.6 49 35 06 17.07 ... 1.11 3 23 15.4 49 22 14 16.42 1.74: 1.48 3 23 22.6 49 29 41 16.72 ... 2.42 3 23 29.9 47 43 20 18.24 ... 0.98 3 23 28.2 47 43 25 18.64 ... 1.00 3 23 30.8 47 42 58 18.68 ... 1.00 3 23 29.1 47 42 43 18.33 ... 1.62 3 23 31.9 49 07 46 14.62 0.89 0.88 3 23 34.0 49 13 34 12.97 0.98 0.91 3 23 49.1 49 16 24 12.44 0.79 0.60 3 23 49.3 49 05 41 19.86 ... 1.68 3 24 03.3 47 37 07 17.65 ... 2.71 3 24 05.9 48 31 29 17.48 .._ 2.66 3 24 09.8 48 45 57 17.66 ... 2.77 3 24 11.9 49 08 54 19.24 ... 2.16 3 24 19.2 49 13 19 8.98 0.34 0.16 3 24 17.8 49 12 23 10.24 0.26 0.07 3 24 15.1 49 12 30 11.90 0.68 0.61 3 24 21.0 49 04 57 15.21 ... 1.03 3 24 25.6 49 14 53 16.30 ... 1.53 3 24 24.4 49 14 54 17.02 ... 1.16 3 24 33.4 49 43 15 12.07 0.82 0.75 3 24 32.6 48 01 19 17.89 ... 1.20 3 24 34.9 48 00 55 16.97 ... 1.20 3 24 32.6 48 00 37 13.50 ... 0.76 3 24 36.8 50 00 14 11.06 0.90 0.87 3 24 43.5 49 53 15 12.58 0.92 0.87 3 24 42.8 49 52 57 15.80 1.07 1.19 3 24 59.8 48 53 02 11.91 0.86 0.91 3 24 59.3 48 37 11 14.72 ... 1.64 3 24 58.7 48 37 11 16.84 ... 1.17 3 25 10.9 49 04 18 16.83 1.55 2.46 3 25 28.7 48 57 52 13.43 0.97 0.93 3 25 50.5 48 59 34 14.24 1.58 1.51 3 25 51.4 48 59 15 14.00 1.51 1.47 3 26 58.7 48 42 31 15.95 ... 0.55 3 27 01.1 48 42 39 16.33 ... 1.03 3 27 06.8 49 41 40 13.30 0.89 0.77 3 27 26.4 47 03 29 9.7 . . . . . . 3 27 33.6 47 55 25 15.25 ... 2.45 3 28 10.4 48 56 08 14.92 ... 1.39 3 28 13.3 48 56 16 18.06 ... 2.96 3 28 11.5 48 55 51 17.07 ... 0.45 3 28 17.2 48 40 58 13.26 1.06 1.05 3 28 16.4 48 40 56 16.76 ... 2.67 3 28 16.4 48 41 02 17.79 ... 1.03 3 28 35.9 49 25 11 17.48 ... 2.71 3 28 38.0 47 55 27 17.99 ... 2.81 3 28 35.0 47 55 05 17.87 ... 2.82 3 28 39.9 48 46 20 17.37 ... 2.63 3 28 43.4 49 29 02 13.38 0.74 0.64 3 28 48.7 48 20 2G 16.92 ... 2.52 3 28 54.3 47 29 19 12.75 0.99 0.98 3 28 53.4 47 30 09 13.87 1.41 1.42 3 29 00.2 49 44 12 13.28 1.09 1.09 3 29 06.2 49 19 14 17.54 ... 2.76 3 29 36.7 48 17 43 10.0 . . . . . . 3 29 50.3 48 06 36 17.32 ... 2.55 3 30 13.0 48 04 28 18.38 ... 1.02 3 30 13.2 48 04 17 18.00 ... 0.73 3 30 12.3 48 04 35 19.77 ... 1.79 3 30 16.9 47 55 25 9.58 1.13 0.95 3 30 33.1 48 30 39 20.01 ... 0.94 3 30 33.2 49 27 22 12.70 0.96 0.88 3 30 41.8 49 03 03 11.89 0.81 0.70 3 31 02.3 48 00 11 18.36 ... 2.92 3 31 02.1 47 59 58 16.04 ... 1.52 3 31 16.0 48 25 14 16.10 ... 2.64 3 31 33.0 47 51 46 6.75 -.02 ... 3 31 43.8 48 06 36 16.55 ... 1.00

222 0.82 101 1.64 313 1.15 111 1.49

313 1.38 111 1.19 111 0.71

202 ... 414 ... 202 1.54 101 ... 101 ... 101 ... 101 ... 212 ... 222 0.88 111 ... 101 ... 101 1.59 101 1.58 202 1.60 202 ... 111 0.24 222 ...

202 ... 202 ... 202 ... 111 0.72 101 ... 101 ... 101 ... 111 0.80 111 0.82 111 ... 322 0.76 101 1.36 101 -.. 424 1.45 111 ... 211 1.48 211 1.41 101 ... 101 ... 111 ...

101 1.55 101 ... 101 1.64 101 ... 222 0.96 202 1.58 101 ... 101 1.59 101 1.61 101 1.61 101 1.58 111 ... 101 1.56 111 0.89 111 1.31 111 0.99 101 1.60

101 1.57 101 ... 101 ... 101 ... 111 ..' 101 ... 111 0.86 111 0.71 101 1.63 101 ... 101 1.58 ... -.12 101 ...

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

29.77 33.09 -3.32 29.48 32.01 -2.53 29.83 32.77 -2.94 29.83 32.66 -2.83

29.24 31.83 -2.59 29.79 33.07 -3.28 29.73 33.37 -3.64

. . . . . . . . .

. . . . . . . . .

. . . . . . . . . 28.99 32.12 -3.13 . . . . . . . . . . . . . . . . . .

. . . . .

. . . . . . . . . 29.41 32.96 -3.55

. . . . . . . . . 29.38 31.88 -2.50 28.99 31.92 -2.93 29.24 31.90 -2.66

29.33 34.50 -5.17 . . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . . 30.20 33.28 -3.08

. . . . .

. . . . . . . . . 30.35 33.70 -3.35 30.21 33.09 -2.88 . . . . . . . . . 30.14 33.34 -3.20 28.95 32.61 -3.66

29.40 32.09 -2.69

28.92 32.75 -3.83 28.92 32.83 -3.91

. . . . . . . . .

. . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . . 29.66 32.72 -3.06 . . . . . . . . . 28.99 31.83 -2.84 . . . . . . . . . 29.94 32.88 -2.94 29.94 32.22 -2.28

29.07 31.94 -2.87 29.12 31.78 -2.66 29.12 31.84 -2.72 28.74 31.95 -3.21

28.84 32.08 -3.24 29.68 33.04 -3.36 29.68 32.87 -3.19 29.56 32.87 -3.31 28.99 31.94 -2.95

28.74 31.94 -3.20

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . . 9.88 33.04 -3.16 30.11 33.35 -3.24 29.14 31.69 -2.55

29.22 32.47 -3.25 28.99 35.76 -6.77

. . . . . . . . .

. . . . . . . . .

Y Y Y

Y? N Y Y Y N N N Y N N N N N Y N N Y Y Y N Y N N N N N Y N N N ? Y N Y Y N Y N Y Y N N N N Y N Y N Y Y N Y Y Y Y N Y Y Y Y Y N Y N N N N N Y Y Y N Y Y? N

. . . . . . Y 4.9 . . . . . . . . . . . . . . . . . . Y 4.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Y 3.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N .__ Y 9.9 Y 8.8 Y 6.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Y 1.2

Y 3.4 . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . . Y 4.5

Y 6.9 N ...

Y ...

Y 6.2 Y ... Y ... Y 4.9

Y 3.1

N .._

Y 8.2

. . . . . .

. . . . . .

. . . . . .

. . . . . .

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. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . . Y 5.3

Y 6.5 . . . . . .

. . . . . .

. . . . . .

... - 2 . 1 3 -3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +4. . . . . . . ... + 1 . 2 0 ... - 2 . 1 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . -5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .__ -6. 12 ... - 4 . 1 6

+6. . . . . . . +5. . . . . . . +3. . . . . . .

... -1. 44:

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . 55:

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

... -37. 24

. . . . . . 50:

. . . . . . . . .

. . . . . . . . . +4. . . . . . .

-4. . . . . . . ... +30. 14 ... -82. 20 ... -12. 12

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

... -11. 13

+4. . . . . . .

-1. . . . . . .

... - 1 . 1 4

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . . +4. . . . . . . . . . . . . . . . . . . . . . . . . +18. . . . . . . ... + 1 . 1 0 +13. . . . . . . ... - 2 . 2 1

... +5. <10 +8. . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

... +7. <10

... -2. 38:

... + 1 . 2 6 +6. . . . . . .

-2. . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

Y Y ... ... ... ?

Y ? Y ... ... ... Y ... ... ... ... ... Y? Y

? ? ?

Y

...

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...

...

...

...

...

...

...

...

... ? ... ... ... ?

Y N N ?

...

...

... N?

?

Y

Y

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...

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Y? ? Y

? ?

...

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Y Y Y Y? N

Y? Y Y N N N Y N N N N N Y N N

Y? Y? Y? N Y N N N N N Y N N N ?

Y? N

Y? Y? N Y N

N? Y? N N N N Y? N Y N Y Y N

Y? Y Y

Y? N

Y? Y ?

Y ? Y? N

Y? N N N N N Y

Y? Y? N Y Y? N

206 Astron. Nachr. 319 (1998) 4

Table 2. Optical counterparts (Continuation)

APX Star RA DEC V B-V V-IK vbi B-V, log log Phot Ha Ha H a Echelle Vrad Final (2000) LX Lbol Lx/Lbol Mem Emiss EW(A) vrad vrad vsini Mem Mem

65 66 67 68 G9 69 70 71 72 73

... . . . . . . . . . . . . APX65B 3 31 43.5 48 06 34 16 .52 1.00 101 N . . . . . . . . . . . . . . . ... N APXGG 3 32 09.6 48 04 20 12.74 1.02 1.03 111 0.92 29.60 33.05 -3.45 Y . . . . . . ... +14. 14 N? ? HE973 3 32 09.5 49 26 22 11.62 0 .89 0.80 111 0.79 30.06 33.47 -3.41 Y . . . . . . ... -1. 12 Y Y APX68 3 32 13.2 49 14 24 11.99 1.45 1.71 111 N . . . . . . ... +11. 19 N? N APX69A 3 32 38.2 47 46 34 16.25 . . . 2.52 101 1.56 29.16 32.35 -3.19 Y Y 6.4 +8. . . . . . . ? Y? APX69C 3 32 40.5 47 46 18 14.71 ... 1.57 101 1.34 29.16 32.59 -3.43 Y N ... . . . . . . . . . ... ? HE1047 3 33 20.4 47 57 03 6 .7 N . . . . . . . . . . . . . . . ... N APX71 3 33 29.1 48 07 15 13.02 0 .93 0 .89 111 0.83 29.27 32.91 -3.64 Y . . . . . . ... -25. <10 N ? APX72 3 33 31.6 48 02 40 12.96 0 .95 0.94 111 0.85 29.36 32.94 -3.58 Y . . . . . . ... +4. 16 ? Y? APX73A 3 36 58.5 47 38 33 14.52 1.44 1.40 111 1.34 30.51 32.60 -2.09 Y? _ _ _ .,. . . . . . . . . . ... Y ?

. . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

Footnotes to Table 2: a) galaxy, b) SB1 (Appendix A)

Many of the optical counterparts to the APX sources appear to have good qualifications for membership based on their photometry and either presence of H a emission or radial velocity near the cluster mean of N -3 km s- l . Among narrow-lined echelle spectra, stars with radial velocities more than 10 km s-l off the cluster mean were considered nonmembers or likely nonmembers on the basis of Vrad. The prevalence of Ha emission among the faint (V> 15) photometric candidates supports membership, though there could still be some contamination by active field stars - this contamination possibility suggested in a few cases by a H a radial velocity measurement that is significantly off the cluster mean. For APX43B and APX45A/B, the presence of Ha emission is noted but equivalent widths were not measured due to weak signal-to-noise in the spectra. In some cases the echelle radial velocity observed for some stars is discordant with membership (i.e. APX32, APX35A for example) and suggests non-membership for such cases. I t is also noticed that the hardness ratios listed in Tab. 1 are generally consistent with those listed by Randich et al. for known a Per members. Overall, 40 out of 73 sources have optical counterparts which are classified as members or candidate members (Y/Y?) of a Persei on the basis of their photometry, while the radial velocities of about 20 of these are also in good agreement with membership. As to most of the remaining objects, though radial velocities may be up to 10 km s-l off the cluster mean, we cannot at this stage exclude membership, since the errors in vrad are rather large and monitoring for binarity should be carried out. For this reason, these objects were flagged with a T?” final membership flag.

It may well be asked as to why the new candidate cluster members found in X-rays were not previously recovered in the Schmidt plate proper motion surveys in this cluster (Stauffer et al. 1985, 1989; Prosser 1992). One reason for this is the effects of image blending with other nearby stars or with plate defects which would cause rnismeasurement of the star’s proper motion and photographic photometry. Ghost images on the plates due to bright stars in the cluster region also can affect both the photographic photometry and proper motion measures for stars within such images, causing them to be rejected in either the photographic photometry or proper motion selection of candidates. As has been demonstrated in other clusters (i.e. NGC 6475 - Prosser et al. 1995, IC2602 - Randich et al. 1995) the ability to identify new candidate cluster members based on their X-ray emission can be a useful tool in investigating the low-mass membership in open clusters.

4.2.

Our photometric and spectroscopic observations have shown that more than half of the X-ray sources have optical counterparts with photometry consistent with cluster membership, and that several of these are most likely cluster members, as indicated by radial velocities/Ha emission. In the following, we will discuss some of the properties of all the photometric candidates and compare them to those of previously known cluster members. We note, however, that additional spectroscopic observations (particularly high dispersion ones) are required to definitively assess membership for stars which either do not have any radial velocity measurements or have V,,d somewhat off the cluster mean.

Properties of the new Alpha Per candidate members

4.2.1.

As mentioned above, most of new cluster candidates lie in the 12 < V < 14 and V> 16 magnitude ranges, or in the late-G to early-K and M dwarf spectral ranges. The fewer number of objects in the 14-16 mag range may indeed reflect the presence of the dip at Mv=8 in the cluster luminosity function which was noticed by Prosser (1992).

We believe that, in addition to these new cluster candidates, there might be additional members which were neither picked out by proper motion surveys, nor detected in X-rays. The detection rate among previously known cluster members was not 100 per cent, and thus, assuming that the X-ray and proper motion surveys are inde- pendent, we expect that a part, of the cluster population which was not selected as members by proper motions,

Color distribution of new candidates

C.F. Prosser and S. Randich: New low-mass members of the a Persei cluster. I. 207

o a Persei

+ APXstar

: 5 6

u 0

3

c

- .- E 4 a I

2

Fig. 2: Distribution of Ha equivalent widths vs. V-IK for a Per members (open circles) and APX members and 0

1 1.5 2 2.5 3 3.5 v-ll candidate members (crosses).

also has X-ray luminosity levels below the sensitivity threshold of the raster X-ray survey. We can estimate the number of these possible additional cluster members in the following way. Assuming that the detection rate among a given sample of objects is the same when considering only previously known cluster members and the whole sample previously known plus new cluster candidates, one can write:

with D R = the detection rate, Ndk = previously known objects detected in X-rays, Nk = the total number of previously known objects, Ndn = the new cluster candidates detected in X-rays, and N , = the total number of new cluster candidates. From equation (1) we get:

or, the number of still undetected new candidates, Nu,, is given by:

Using the same color subdivision of Randich et al., our sample of 40 photometric candidates is composed of N 18 K-dwarfs, N 18 M-dwarfs, and four earlier-type objects. Considering the whole raster area, Randich et al. obtained a detection rate of 79 and 35 per cent for K and M dwarfs, respectively. By using equation (3), we thus obtain that about five new K-type cluster candidates and 33 new M dwarfs should still be present in the cluster area covered by raster scan, besides the ones which were detected. If this is correct, this means that the number of previously unknown late-G/K and M dwarfs is comparable to that of previously known objects at least in the area surveyed by the raster scan.

4.2.2.

Rotational velocities have been obtained for about half of the photometric candidates. The sin i values listed in Table 2 are within the range of rotational velocities measured for known a Per members (e.g., Prosser 1992), although we notice the absence of very rapidly rotating objects (with wsini 2 100 km s-l) among the newly identified cluster candidates. As to Ha emission, we plot in Figure 2 Ha equivalent widths as a function of V-IK for known a Persei members and for the APX candidates. The figure shows that there are not major differences between the two samples, nor are there APX stars which are either particularly weaker or stronger than known members. Such a result gives strength to the hypothesis of membership of the APX candidates.

In Fig. 3 we plot the logLx vs. B-V, distribution for the raster survey of Randich et al. (1996), together with the APX stars considered to be members and candidate members in Table 2. Cases of multiple stars associated with the same X-ray source are connected by a line. The APX stars are primarily concentrated in the late-G to M dwarf range and their distribution in logLx is consistent with the distribution among catalogued cluster members

Rotational velocities, Ha emission and X-ray emission

208 Astron. Nachr. 319 (1998) 4

Raster a

30.5 + -I 3 0 1 '

Fig. 3: logLx vs. B-V, distribution of cy Per members from the raster survey (open circles, upper limits), together with the new APX members and can- didate members from the present study

A0 FO GO KO K 5 MO (crosses). Cases of multiple stars asso- ciated with the same x-ray source are 28 " l L 1 ' l l ' " " " l " " l l l l " l l l ' l l l l ' l l l ' l ' l

- 2 0 2 .4 .6 .8 1 1 2 1.4 1 6 1 8 B-V. connected by a line.

in the raster survey. APX73 is somewhat discordant from the rest of the sample; its log(LX/Lb,,l) value is N -2, indicating that it either had a flare event or is an active field star.

We have also constructed the X-ray luminosity distribution function (XLDF) for the total sample of previously known M dwarfs (only those comprised in the raster region) plus the new M photometric candidates identified here from the raster scan. The XLDF for this global sample does not noticeably differ from the one obtained by Randich et al. considering only known members, although the global sample appears slightly more X-ray luminous, as one would expect, since the new candidates are X-ray selected.

5 . Summary

Using photometric and spectroscopic observations, analysis of the 73 remaining X-ray sources from the raster survey of Randich et al. (1996) has yielded N 40 new candidate cluster members in the late-G to M dwarf range. The w sini and Ha emission properties of these stars have been found to be consistent with cluster membership. These new members and candidate members represent an important new addition of solar-type and low-mass pre-main sequence (PMS) stars which may be used to investigate the evolutionary properties of PMS stars as they evolve to the zero-age main sequence or ZAMS. It is estimated that an additional N 40 members within the 10 sq. deg. region of the raster survey exist, but have remained undetected at the current X-ray emission sensitivity levels. Finally, N 30 X-ray sources do not appear to have optical counterparts which are candidate cluster members. Limited data prevents detailed analysis of these cases, which most likely are comprised of a selection of extragalactic objects and field stars in our own Galaxy.

Acknowledgements. Support for this research has been provided by NASA Grant Nos. NAGW-2698 and NAG5-2171. S. Randich acknowledges the support of a MPG fellowship. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAGW-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present conipressed digital form with the permission of these institutions. The National Geographic Society - Palomar Observatory Sky Atlas (POSS-I) was made by the California Institute of Technology with grants from the National Geographic Society. The Oschin Schmidt Telescope is operated by the California Institute of Technology and Palomar Observatory.

A Finding charts

In this appendix we provide finding charts at the X-ray position for the new 'APX' sources discussed in the text (re. Table I ) , which were not associated with bright stars/previously known cluster members. Charts are approximately 3 x 3 arcmin, North up and East to the left, and are extracted from a digitized scan of the Palomar Schmidt XE plates using the GASP software at Space Telescope Science Institute. The circles are centered at the position of the X-ray detection and have a radius of N 30 arcsec. An offset distance of 30 arcsec turns out to be a reasonable offset within which one may expect to

C.F. Prosser and S. Randich: New low-mass members of the a Persei cluster. I. 209

recover optical counterparts for the raster X-ray sources (see sect. 3.2.1 of Randich et al. 1996). Individual notes on the finding charts follow: APX3: all objects are photometry nonmembers.

A) ( a , 6) = ( 3 21 24.8, 49 57 24). (V,B-V,V-IK) = (15.60, 1.62, 1.58). B) (a , 6) = (3 21 26.9, 49 57 09). (V,B-V,V-IK) = (17.23, 0.94, 0.83). C) (a , 6) = ( 3 21 29.8, 49 57 15). (V,B-V,V-k) = (15.66, 0.86, 0.89).

APX4: HE497 = BD48 893. Proper motion nonmember. APX8: HE553 = BD48 898. Proper motion nonmember. APX15: optical binary, faint companion has (V,V-IK) = (15.87, 1.03), phot. nonmember. APX17: B) (a, 6) = (3 24 00.7, 47 36 57). (V,V-IK) = (16.69, l.ll), phot. nonmember.

APX20: possibly nebulous object. APX21: The reported echelle data for HE606 in Table 2 are from a KPNO 4m echelle spectrum obtained during Nov. 1994

(Prosser 1996). APX22: B) (a , 6) = (3 24 22.9, 49 04 32). (V,V-IK) = (17.49, 1.14), phot. nonmember.

C) ( a , 6) = (3 24 22.3, 49 04 23). (V,V-IK) = (18.09, 1.33), phot. nonmember. APX23: C) ( a , 6) = (3 24 25.5, 49 14 21). (V,V-IK) = (17.25, 1.56), phot. nonmember. APX26: HE615 is a proper motion nonmember (Heckmann et al. 1956), though it appears as a possible photometric binary

member based on the CCD photometry in Table 2. It has been identified as a single-line spectroscopic binary from comparison with a KPNO 4m echelle observation obtained during Nov. 1994 which finds 'U,,d = -20 km/s (Prosser 1996).

A) (a , 6) = (3 25 00.5, 48 48 22). (V,V-IK) = (19.94, 1.38), galaxy?. B) (a , 6) = (3 24 58.8, 48 48 15). (V,V-IK) = (18.74, 1.17). C) (a , 6) = (3 24 59.6, 48 48 01). (V,V-IK) = (16.57, 1.11). D) (a , 6 ) = (3 24 58.1, 48 48 01). (V,V-IK) = (16.10, 0.99). E) (a , 6) = (3 24 57.9, 48 48 04). (V,V-IK) = (16.94, 0.86).

A) (a , 6) = (3 25 31.5, 47 49 08). (V,V-IK) = (16.46, 0.57). B) (a , 6) = (3 25 28.2, 47 49 20). (V,V-IK) = (17.50, 1.10). C) (a , 6) = (3 25 30.1, 47 49 14). (V,V-IK) = (18.60, 1.29). D) (a , 6) = (3 25 30.1, 47 49 06). (V,V-IK) = (18.35, 1.04). E) (a , 6) = (3 25 35.1, 47 49 26). (V,V-IK) = (14.99, 1.44).

A) (a , 6) = (3 25 50.6, 48 39 51). (V,V-IK) = (18.00, 1.05). B) ( a , 6) = (3 25 52.6, 48 39 17). (V,V-IK) = (19.63, 1.64). C) ( a , 6) = ( 3 25 51.8, 48 39 00). (V,V-IK) = (19.17, 1.19).

C) (a , 6) = (3 24 00.8, 47 36 29). (V,V-IK) = (18.36, 1.43), galaxy.

APX29: all objects are photometry nonmembers.

APX33: all objects are photometry nonmembers.

APX34: all objects are photometry nonmembers.

APX35: C) (a , 6) = (3 25 55.9, 48 58 59). (V,V-IK) = (15.79, 0.89), phot. nonmember. APX36: all objects are photometry nonmembers.

A) (a , 6) = (3 25 54.2, 49 09 30). (V,B-V,V-IK) = (12.50, 0.62, 0.53). B) (a , 6) = (3 25 55.3, 49 09 11). (V,V-IK) = (16.71, 1.70). C) (a , 6) = (3 25 57.2, 49 09 16). (V,B-V,V-IK) = (15.85, 1.30, 1.51). D) (a, 6) = (3 25 54.0, 49 08 45). (V,V-IK) = (18.18, 0.74).

at x-ray position has (V,V-IK) = (17.80, 1.21), phot. nonmember.

A) ( a , 6) = (3 27 45.1, 48 16 20). (V,V-IK) = (16.97, 1.02). B) ( a , 6) = (3 27 44.5, 48 16 16). (V,V-IK) = (18.13, 1.47), galaxy?. C) (a , 6) = (3 27 44.6, 48 16 10). (V,V-IK) = (18.69, 0.94). D) (a , 6) = (3 27 43.9, 48 15 55). (V,V-IK) = (18.53, 1.22). E) (a , 6) = (3 27 46.1, 48 15 58). (V,V-IK) = (19.19, 1.27).

APX39: source is at extreme edge of field, HE749 (=BD46 755, proper motion nonmember) is likely counterpart. Faint star

APX41: all objects are photometry nonmembers.

APX44: A) (a , 6) = (3 28 34.0, 49 25 06). (V,V-IK) = (14.34, 0.80). phot. nonmember. B) ( a , 6 ) = ( 3 28 35.2, 49 25 35). (V,V-IK) = (15.61, 0.68). phot. nonmember.

APX49: both objects are photometry nonmembers. B) (a , 6 ) = (3 28 56.1, 47 29 20). (V,B-V,V-IK) = (15.09, 1.08, 0.99).

APX51: A) (a , 6) = (3 29 06.3, 48 03 30). (V,V-IK) = (16.35, 1.59), phot. nonmember. B) ( a , 6) = (3 29 08.2, 48 04 04). (V,V-IK) = (16.30, 0.78), phot. nonmember.

APX52: B) ( a , 6) = (3 29 04.0, 49 18 39). galaxy. APX53: HE860 = BD47 838, proper motion nonmember. APX54: all objects are photometry nonmembers.

D) (a , 6) = (3 28 54.1, 47 30 09). (V,B-V,V-IK) = (14.97, 0.90, 0.91).

A) (a, 6) = (3 29 42.4, 48 34 48). (V,V-IK) = (18.45, 1.24).

210 Astron. Nachr. 319 (1998) 4

B) ( a , 6) = (3 29 41.8, 48 34 31). (V,V-IK) = (19.63, 1.20). C) (N, 6) = (3 29 42.2, 48 34 16). (V,V-IK) = (17.75, 0.99).

APX55: HE879, which lies near the source, is a proper motion candidate member. APX57: HE888 is a proper motion nonmember. The reported echelle data for HE888 in Table 2 are from a KPNO 4m

echelle spectrum obtained during Nov. 1994 (Prosser 1996). No significant lithium feature is observed for HE888, thus it is considered a nonmember from spectroscopic observations.

APX60: all objects are photometry nonmembers. A) (CY, 6) = (3 30 34.9, 48 19 52). (V,V-IK) = (16.81, 1.99), VB 2". B) ( C Y , 6) = (3 30 32.1, 48 19 22). (V,V-IK) = (15.20, 0.90). C) (a , 6) = (3 30 34.3, 48 19 14). (V,V-IK) = (18.36, 0.88).

Faint object south of HE955 has (V,V-IK) = (15.22, 1.08), phot. nonmember. APX64: HE955 = BD47 846s, a proper motion candidate member.

APX65: AB close double, separation 2". APX67: No proper motion measure (Heckmann et al. 1956) is available for HE973. A strong lithium feature is evident

however in a Nov. 1994 KPNO 4m echelle observation of HE973 (Prosser 1996), further supporting cluster mem- bership.

APX69: B) ( ( 2 , 6) = (3 32 38.9, 47 46 25). (V,V-IK) = (16.44, 1.09), phot. nonmember. D) ( t ~ , 6) = (3 32 40.9, 47 46 05). (V,V-IK) = (12.86, 1.68), phot. nonmember.

APX70: HE1047 = BD47 850, proper motion nonmember. APX73: source is at extreme edge of raster survey, resulting in a larger error in the position determination.

B Special cases

In this appendix we discuss a few stars/sources of interest. We note the recovery here of catalogued stars for APlOl and HE955 which were not listed in the raster survey. These stars were not identified in the raster survey because they lie slightly beyond the 30 arcsec cutoff radius used there. For HE606, there are two other stars near the X-ray position. Although these two stars are cluster nonmembers while HE606 is considered to be a cluster member, the identification of HE606 as the optical counterpart to the X-ray source requires further study. Detected in the raster survey, AP76 (=APX48) was recovered again here and we have retained its listing in Table 1 due to the new photometric and spectroscopic data obtained in the present study.

In some cases, relatively bright nonmember HE stars (Heckmann et al. 1956) have been found to be associated with an X-ray source. The HE nonmembers so detected in X-rays are likely to be relatively nearby field stars. Finally, in some instances in Table 2, we find a galaxy near the X-ray position (ie. APX12, APX43, APX56). Particularly when the other stars near the X-ray source are photometric nonmembers to the a Per cluster, the possibility that the X-ray emission is of extragalactic origin should be considered.

References

Bessell, M.S., Weis, E.W. 1987, PASP, 99, 642 Cruddace, R., Hasinger, G., Schmitt, J.H.M.M. 1988, in: F. Murtagh (ed.): Proc. ESO Conf. on Large Databases, p. 177 Gray, D.F. 1976, The Observation and Analysis of Stellar Photospheres. Wiley, New York Harris, D.L. 1963, in: Strand (ed.): Basic Astronomical Data. University of Chicago Press, Chicago, p. 263 Heckmann, V.O., Dieckvoss, W., Kox, H. 1956, Astron. Nachr. 283, 109 Joner, M.D., Taylor, B.S. 1990, PASP, 102, 1004 Landolt, A. 1992, AJ, 104, 340 Latham, D.W., Stefanik, R.P. 1991, in: Reports on Astronomy, IAU Transactions XXIB, Kluwer, Dordrecht, p. 269 (CfA

Marcy, G.W., Benitz, K.J. 1989, ApJ, 344, 441 Monet, D.G., Dahn, C.C., Vrba, F.J., Harris, H.C., Pier, J.R., Luginbuhl, C.B, Ables H.D. 1992, AJ, 103, 638 Prosser, C.F. 1992, AJ, 103, 488 Prosser, C.F. 1996, private communication Prosser, C.F., Stauffer, J.R., Caillault, J.-P., Balachandran, S., Stern, R.A., Randich, S. 1995, AJ, 110, 1229 Randich, S., Schmitt, J.H.M.M., Prosser, C.F., Stauffer, J.R. 1995, A&A, 300, 134 Randich, S., Schmitt, J.H.M.M., Prosser, C.F., Stauffer, J.R. 1996, A&A, 305, 785 Schild, R.E. 1983, PASP, 95, 1021 Soderblom, D.R., Pendleton, J., Pallavicini, R. 1989, AJ, 97, 539 Stauffer, J.R., Hartmann, L.W., Burnham, J.N., Jones, B.F. 1985, ApJ, 289, 247 Stauffer, J.R.., Hartmann, L.W., Jones, B.F. 1989, ApJ, 346, 160

preprint No. 3316)

Addresses of the authors:

Charles F. Prosser, NOAO, P.O. Box 26732, Tucson, AZ 85726-6732 USA, e-mail: cprosser@noao.edu Sofia Randich, Osservatorio Astronomico di Arcetri, Largo Enrico Fermi 5, 1-50125 Firenze, Italy,

e-mail: randich@arcetri.astro.it

C.F. Prosser and S. Randich: New low-mass members of the cy Persei cluster. I. 211

212 Astron. Nachr. 319 (1998) 4

C.F. Prosser and S. Randich: New low-mass members of the cy Persei cluster. I. 213

214 Astron. Nachr. 319 (1998) 4