Chandra Observation of

Triggered Star Formation in Cepheus K.V. Getman, L.K. Townsley, P.S. Broos, E.D. Feigelson, G.P. Garmire, M. Tsujimoto (Penn State University)

ABSTRACTWe present the first high-spatial-resolution X-ray images and spectra of the Cepheus B/Sharpless 155 interface region, obtained in a single 30-ksec Chandra/ACIS observation in March 2003. This rich high-mass star-forming region reveals a complex mix of point source and diffuse X-ray emission. The OB association Cep OB3b is resolved at the arcsecond level into more than 450 sources. An embedded stellar cluster, representing the most recent generation of star formation at the S155/CepB interface, is resolved into several components. Soft diffuse X-ray emission (kT=0.8 keV) centered on the O7 star HD 217086 pervades the complex. A remarkably strong X-ray flare is seen in one of the pre-main-sequence cluster members. These and other highlights of the Chandra data will be described. This work is supported by NASA contract NAS8-38252 (G.P. Garmire, PI).

<= DSS Optical image of Cep B, ~ 19' x 19'.

* Distance ~ 700 - 900 pc.* Cep B - hottest CO component of Cepheus Molecular Cloud (CMC).* To N-W of Cep B, lies Cep OB3 association (II and I generations with ages of few Myr). * S155 - optical HII region with material driven by O7n star HD217086.* Interface CMC-S155 is delineated by ionization fronts (IF), that may compress CMC and trigger star formation.* Unlike Orion region Cep B is at favorable viewing angle = cloud edge, eroded by radiation, moves eastward with expected newly emergent stars to the west of CMC.

Cep OB3

Cep B

O7n

IF

<= Chandra ACIS-I smoothed image, 0.5-8.0 keV band, ~ 17' x 17', 30 ksec exposure.

* Over 450 X-ray point sources are detected. * Soft (0.8 keV) diffuse X-ray emission around the O7n star is seen.* Source extraction, photometry, variability, and spectral analysis are performed using sophisticated semi-automated IDL package ACIS_Exract: http://www.astro.psu.edu/xray/docs/TARA/ae_users_guide/

* Detection limit for lightly absorbed sources

Lt,lim

~ 1029.2 erg/s, for sources in HII region with

typical NH of (6-7)x1021 cm-2 (Av ~ 3 mag) L

tc,lim

~ 1029.5 erg/s.

Hints for the triggered star formation and existence of the third generation of stars, embedded in the Cep B, or located at the CMC-S155 interface were presented by:WSRT radio continuum study (Felli et al. 1978), FIR and CO (Minchin et al. 1992), optical and NIR (Moreno-Corral et al. 1993), VLA and NIR study (Testi et al. 1995), ROSAT X-ray (Naylor & Fabian 1999) and FIR, radio, molecular line data (Sridharan et al. 2002; Beuther et al. 2002), resulted in detection of 3 radio, 3 NIR, 2 X-ray and 1 FIR (HMPO) objects related to the CMC-S155 interface region.

Combination of high sensitivity and special resolution Chandra X-ray and 2MASS NIR data is an excellent tool to search for new regions of star formation activity.

X-ray emission from pre-main sequence stars (PMS):* is 1 - 4 orders of magnitude stronger than typical main sequence levels,* is mostly uncorrelated with the presence or absence of an IR-excess disk,* often suffers less background-foreground contamination,* confirms the young stellar object (YSO) status of optical/IR objects,* penetrates deeper than IR into molecular cloud cores.

<= About 20% (~90 of ~450) of all detected X-ray sources have median energies > 2 keV = suffer high local absorptions (marked by circles on ACIS-I raw 4 pixels per bin image). Among them ~ 30 (blue circles):* are highly clustered,* lie at the CMC-S155 interface region,* include previously known 3radio + 3NIR + 2 X-ray objects, mentioned above (although HMPO object still remains undetected in X-ray).

Of those 30 cluster members 80% have NIR 2MASS counterparts (blue circles on CC and CMD diagrams). The CC and CMD diagrams (=>) indicate that:* most of the members of the discovered cluster have K-excess, omen of disk = omen of youth, * they are low-mass YSOs with masses of 0.1 - 2 solar mass,* they naturally suffer higher absorptions than the rest of the detected X-ray population (grey circles), low-mass members of the older Cep OB association residing in the HII region with the typical absorption of ~ 3 mag.

0 1 2 3H - K

s [mag]

0

1

2

3

J - H

[mag

]

Av = 0 mag

Av = 5 mag

0 1 2 3J - H [mag]

-8

-6

-4

-2

0

2

4

6

8

10

abso

lute

J [m

ag]

4 MO

Av = 10 mag

2 MO

0.2 MO

1 Myr

ZAMS

<= Chandra ACIS swiss-cheese (point sources-removed) raw image (4 pixels per bin). Solid line delineates the extraction area of the diffuse emission around O7n star, dashed line - the background.

To examine the nature of the diffuse X-ray emission within the HII region, we compared its spectrum with that of the point sources.

<= ACIS spectrum of the diffuse emission. It is reasonably well represented by a two-temperature thermal plasma with a single absorption:

NH1

~ 0.69 (+0.16 -0.14) x 1022 cm-2

kT1 ~ 0.76 (+0.18 -0.13) keV

NH2

~ 0 cm-2

kT2 ~ 0.20 (+0.06 -0.06) keV

Observed and absorption-corrected luminosities (assuming D ~ 725 pc):L

t ~ 1031.3 erg/s

Ltc ~ 1031.9 erg/s

<= ACIS spectrum of the combined point sources is well represented by a two-temperature thermal plasma with a similar absorption:

NH ~ 0.63 (+0.06 -0.07) x 1022 cm-2

kT1 ~ 0.60 (+0.05 -0.05) keV

kT2 ~ 4.58 (+0.52 -0.34) keV

Observed and absorption-corrected luminosities (assuming D ~ 725 pc):L

t ~ 1032.7 erg/s

Ltc ~ 1033.0 erg/s

Nature of the diffuse emission: origin from O star wind versus soft component of the undetected point source population? Pro/cons of O star wind:

Pros:* Spectrum of the diffuse emission of Cep B is similar to that of M17 (Townsley 2003).* O7n star HD217086 has a high wind velocity.

Cons:* Spectrum of the diffuse emission is similar to the soft component of the point source spectrum (see below).* COUP XLF function (Getman et al. 2005) can explain the observed brightness (L

t) of

diffuse emission.