ROSAT observations of Cepheus OB3: the discovery of low-mass stars

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ROSAT observations of Cepheus OB3: the discovery of low-mass starsTim Naylor1 and A. C. Fabian21Department of Physics, Keele University, Keele, Staffordshire ST5 5BG2Institute of Astronomy, Madingley Road, Cambridge CB3 OHAAccepted 1998 September 25. Received 1998 September 21; in original form 1996 December 18A B S T R A C TWe present X-ray images of the high-mass star-forming region Cepheus OB3, in which wediscover over 50 point sources, the majority of which are T Tauri stars. We use the ratio of high-mass to low-mass stars to constrain the initial mass function, and nd that it is consistent withthat for eld stars. This supports the picture that there is little difference between high-massand low-mass star-forming regions. The presence of low-mass stars between the high-massstars and the molecular cloud they are ionizing argues against mechanisms whereby heating ofthe cloud inhibits low-mass star formation. Finally, the efciency of star formation in thisregion is shown to be of the order of a few per cent.Key words: stars: formation stars: luminosity function, mass function stars: pre-main-sequence open clusters and associations: general open clusters and associations:individual: Cepheus OB3 X-rays: stars.1 I N T R O D U C T I O NThe observational distinction between the high- and low-mass star-forming regions is gradually being eroded. The Orion OB associa-tion, traditionally viewed as a high-mass star-forming region, haslong been known to harbour low-mass T Tauri stars, and Walter et al.(1994) have discussed the low-mass stars in the Upper Sco associa-tion (Sco OB2-2). Recently, Schulz, Berghofer & Zinnecker (1997)and Gregorio-Hetem et al. (1998) have also presented X-rayobservations of distant OB regions which seem to show thepresence of T Tauri stars. The data presented here conrm thisview, with the discovery of low-mass stars in another OB region,Cepheus OB3. Walter & Boyd (1991) have shown that the com-plementary case, that low-mass, or T associations, should containhigh-mass stars is probably correct, at least for Taurus-Auriga.If there is no physical distinction between these two types of star-forming region, then it would seem natural to assume that there isjust one star formation mechanism which operates within them. Thestrongest evidence in favour of such a mechanism would be auniversal initial mass function (IMF), although if there weredifferent mass functions, this would not necessarily rule out asingle star formation mechanism. Despite the large number of mea-surements now available, there is no strong evidence against aubiquitous IMF for stars greater than 1 M(, although the evidencebecomes weaker as the IMF becomes more structured below this mass.Furthermore, measurements that span a large range of mass, particu-larly those that cover both the late-type and early-type stars, are rare.2 T H E C E P H E U S O B 3 A S S O C I AT I O NThe Cepheus molecular cloud is clearly visible on sky survey platesas a region of low stellar density. The outlines traced by the starsmatch the J 1 ! 0 12CO maps of Sargent (1977, 1979) well. Themolecular cloud is structured, with a strong CO peak at its mostnorth-western point, known as Cepheus B. Outside Cepheus B, tothe north and west of the cloud, lies the stellar association CepheusOB3 (see Fig. 1). It is divided by Blaauw (1964) into two popula-tions, OB3b being younger than OB3a. These ages are, however, thesubject of some controversy, since the turn-off ages are 7.5 and5.5 Myr (Jordi, Trullols & Galad-Enrquez 1996), whilst theexpansion ages are 0.72 and 0.48 Myr (Garmany 1973).There are obvious signs of star formation activity on the edge of themolecular cloud closest to the OB3b association. This north-westernedge of the cloud is bounded by an optical H II region Sh 2-155(Sharpless 1959), in which material is being driven off the cloud bythe O7n star HD 217086 (Panagia & Thum 1981). A trapezium ofstars appears to have recently emerged from the edge of the cloud(Testi et al. 1995), and Cepheus B itself is thought to be in the earlystages of contraction towards becoming a new cluster of stars.Perhaps the most important aspect of Cep OB3 is the veryfavourable viewing angle at which we see the progression of starformation. As the surface of the cloud is being eroded by theradiation from one or more of the early-type stars, the cloud edgemoves eastwards across our eld of view, with the newly emergentstars clear of the molecular cloud and to the west. This is in contrastto the rather awkward view we have of the Orion OB1 region, wherethe stars lie between the cloud and ourselves.Throughout this paper we will use the distance and reddeningsfor Cep OB3b derived by Moreno-Corral et al. (1993) of 900 6 100pc (which is consistent with most measurements) and AV 2:9.3 O B S E RVAT I O N SWe observed a eld centred on the O7n star HD217086 with bothMon. Not. R. Astron. Soc. 302, 714722 (1999)q 1999 RASthe ROSAT PSPC and HRI, a log of the observations being given inTable 1. Fig. 2 shows the two PSPC images, and the HRI image. Thebright source at the centre of all the observations is the O7n star,which lies in the northern section of what appears to be a cluster ofX-ray sources. In addition to the point sources, there is underlyingunresolved diffuse X-ray emission. The Einstein IPC observation ofthis eld (Fabian & Stewart 1983) detected the X-ray emission fromthis cluster, but was unable to resolve it into individual sources.There are virtually no X-ray sources to the south-east, which isunsurprising as this is the region of the molecular cloud (see Fig. 1).There are, however, a few sources to the north and east of the maincluster. The variability of several sources between the two PSPCobservations is obvious, the most spectacular example being thesource to the north-east of the O7n star (Star 45 in Table 2), which,aside from the O7n star, is the brightest object in the rst PSPCobservation, and yet is barely detected in the second (see alsoTable 2). There is almost certainly also variability between thePSPC and HRI observations, but the different passbands of the twoinstruments should make us cautious about such an interpretation ofthe data on a source-by-source basis.We rst created a catalogue of all the X-ray sources we detectedby searching the central 40-arcmin diameter of the HRI and twoPSPC images using the point-source search algorithm of Allan(1992), which is also described briey in, for example, Jeffries,Thurston & Pye (1997). We searched to a level of 4.5j at a givenposition in the PSPC images, and 4.25j in the HRI images, whichsimulation shows should result in less than one spurious source perimage (Jeffries et al.. 1997; Jeffries, private communication). Eachof the source lists contained at least three objects that could beidentied with stars in the HST GSC. This allowed us to carry out abore sight correction, and sum the two PSPC images, so that afurther search for sources could be carried out. As a result of this,the maximum error in either RA or declination between an X-rayposition and a GSC position was 15 arcsec for the combined PSPCimage, and 7 arcsec for the HRI. The bore sight correction wasapplied to each list, before attempting to combine them to producea single catalogue for the whole association. Since many of thesources are variable, we searched for positional coincidencesamongst the lists, ignoring the ux levels of the objects. Sourceswithin 15 arcsec were considered to be the same object. Thisseparation is sufciently large that it may occasionally result intwo sources being assumed to be one object, when in fact they areunrelated. However, in the statistical analysis presented later, it isbetter that this occurred, than that a single object should berepresented twice in our catalogue.Once the lists had been merged, the coordinates for each objectwere taken from the best available source, i.e., the HRI whereavailable, otherwise the combined PSPC image or, in two cases, thesingle PSPC images. These positions are presented in Table 2. Wethen evaluated the ux at each of these positions in all threeobservations, correcting for the gross vignetting, and in the caseof the PSPC, the vignetting due to both the thin and thick wires.Where the ux was either unmeasurable with the point-sourcesearch algorithm, or below the error, we calculated a 1j upperlimit. Both the uxes (with 1j errors) and limits are presented inTable 2.With these uxes, we carried out a more formal variabilityanalysis. First, we looked for sources that varied between the twoPSPC observations at greater than the 99 per cent condence level,nding that Sources 21, 22, 23, 36, 44 and 45 did so. Next, wesearched for variability within each observation for the observationsof sources which had more than 50 detected photons, exceptingSources 6 and 7 which are too close to allow reliable photometry.For each source we extracted a light curve in bins of 1000 s for thePSPC and 2000 s for the HRI, and then tested to see if the resultinglight curves were consistent with a constant ux using a x2 test.Only Source 23 in PSPC1 was inconsistent at a greater than 99 percent condence level. This, and the null results of our other tests, arereported in Table 2. Since the total number of photons detected inthe PSPC1 observation of Source 23 is small, all one can say withcertainty is that the source is brighter at the beginning of theobservation than at the end.To be certain of the nature of these sources, we required spectro-scopy of their optical counterparts. To generate candidates forspectroscopic observations, we rst checked the positions againstthe HST GSC. We report the 10 positive results in Table 2. Themodal magnitude (in 1-mag bins) of the HST GSC in this region is13. Thus we expect that the catalogue of stars outside the region ofour CCD data is complete to about 12th magnitude.The optical ux limit from the GSC is above the ux we wouldexpect for T Tauri stars, and so we required deeper imaging of theelds. Thus we observed the region using the 0.6 m Thorntonreector at Keele Observatory during the night of 1995 December29. The system uses an ST6 CCD camera at the f =4:5 NewtonianROSAT observations of Cepheus OB3 715q 1999 RAS, MNRAS 302, 714722Figure 1. A schematic of the Cepheus OB3b region. The dashed line marksthe lowest CO contour, taken from Sargent (1977); the molecular cloud is tothe lower left of this line. The solid line is the brightest part of the Ha nebulaSh 2-155, and is another way of marking the edge of the cloud. The opencircles are the early-type members of the Cepheus OB3b association takenfrom BHJ and Sargent (1977), with the O7n star BHJ 41 marked. The crossesare the sample of coronal X-ray sources discussed in the text. The dottedcircle is the PSPC inner support ring, and represents the extent of the areasearched for X-ray sources.Table 1. Log of ROSAT observations.Instrument Date start Date end Total exposure Bore sight correctionPSPC 1992 July 29 1992 July 30 4252 s 1:s0, 0500PSPC 1992 December 27 1993 January 6 6357 s 1:s1, 0300HRI 1994 July 21 1994 July 24 20308 s 0:s7, 0100focus, and in our case we used a Johnson V-band lter. We observedfour elds for two min each, covering a region of approximately 14by 17 arcmin2, centred on RA 22h 56:m7, Dec. 628410 (J2000). Theimages were dark-subtracted, and combined into a single imagethat we could use for astrometry. We searched for positionalcoincidences between our X-ray detections and the optical eld.We chose to accept correlations within 7 arcsec of an HRI position,and 15 arcsec of a PSPC position, irrespective of whether thecounterparts were from the GSC or our own data. The results arelisted in Table 2.We measured optical magnitudes for the counterparts using theaperture photometry algorithm described in Naylor (1998). Inaddition to the long exposures, we obtained a single 10-s exposurethat had stars in common with all four long exposures, allowing usto transfer all the data to a single zero-point. Since the shortexposure also contained both Stars 40 and 41 of Blaauw, Hiltner& Johnson (1959, hereafter BHJ), we used their photometry of thesestars to calculate our overall zero-point. We have not attempted anycolour correction, but the correction for this system is comparablewith our quoted accuracy (0.1 mag). The faintest objects visible inthe optical data correspond to about V 16:8, which is an approxi-mately 10j detection. We will use this as the upper limit to theoptical ux for those sources which have empty error boxes. Thechance of a random correlation between a star brighter thanV 16:8 in our CCD images and, say, a 10-arcsec radius errorcircle is highly dependent on position within the association,because the stellar density changes dramatically across the eld-of-view. For the densest parts of the cluster, however, it is around 10per cent. Thus we expect one or two of our 16 identications basedon the CCD imaging alone are spurious.716 T. Naylor and A. C. Fabianq 1999 RAS, MNRAS 302, 714722Figure 2. The X-ray images from observations PSPC1 (upper left), PSPC2 (upper right), HRI (bottom left) and PSPC2 overlayed on the Digitised Palomar SkySurvey (bottom right). The lowest contour is at 0.1 counts per 15-arcsec pixel. Each subsequent contour is at a factor of 2 greater counts than the last. The PSPCimages have been smoothed with the adaptive routine described in Ebeling, White & Rangarajan (1997), and are for the 0.52 keV band.22h 55m 56 57 58 59Right Ascension (2000)62 30 35 40 45 50 5563 00Declination (2000)22h 55m 56 57 58 59Right Ascension (2000)62 30 35 40 45 50 5563 00Declination (2000)22h 55m 56 57 58 59Right Ascension (2000)62 30 35 40 45 50 5563 00Declination (2000)22h 55m 56 57 58 59Right Ascension (2000)62 30 35 40 45 50 5563 00Declination (2000)22h 55m 56 57 58 59Right Ascension (2000)62 30 35 40 45 50 5563 00Declination (2000)22h 55m 56 57 58 59Right Ascension (2000)62 30 35 40 45 50 5563 00Declination (2000)58 00 30 57 00 30 56 00Right Ascension (2000)ROSAT observations of Cepheus OB3 717q 1999 RAS, MNRAS 302, 714722Table2.Catalogueofsources.Countsks1HaLi6708AStarJ2000PositionHRIPSPC1PSPC2VEW(A)EW(A)CommentsOthernames1225418.1+624011(P)