Protostars and V 2005 8425.pdf

Cataloging the Sigma Orionis Region’s Young Stellar Members. N.R. Adams-Wolk1, S.J. Wolk2, F.M. Walter3 and W.H. Sherry4, 1 Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden Street, Cambridge MA, 02138 ([email protected]), 2 CfA, ([email protected]), 3Department of Physics and Astronomy , SUNY at Stony Brook, Stony Brook, NY 11794-3800([email protected]), 4NSO/Tucson, 950 N. Cherry Avenue, Tucson, AZ 85719-4933 ([email protected])

Introduction: In 2002, the Chandra X-ray Obser- almost all have lost their disks, but around Sigma vatory observed the young forming region around Ori the wTTs to cTTs ratio is about two which allows Sigma Orionis with the goal of studying the X-ray us to study both populations. of a well understood sample of pre- stars. The Sigma Ori cluster is uniquely Data: We have the following data for the stars in suited to this study; it spans different stellar interior the Sigma Orionis Cluster. conditions, almost 3 orders of magnitude in mass, it is X-ray: relatively close, almost dust free (5% of the ONC), the • Our HRC observation proper angle to have a reasonable mix of stars with and • An ACIS HETG archive observation without disks and we have measured its mass function • An XMM-Newton archive observation well below the (BD) limit. Optical: • B-I survey [3] We will present a catalogue of X-ray characteristics of • R and I survey [4] known PMS stars in this region using our Chandra : data, archival Chandra and XMM-Newton data, and • The 2MASS survey ground based data. • CIRM JHK pointed observations. Spectral: Why Sigma Orionis?: The Sigma Orionis cluster • 200+ spectra of PMS candidates was selected because it is young (~ 2 Myr) yet clear of dust. Even the moderate NH of the ONC can absorb the Based on deep optical images, and near-IR images, soft X-rays expected from BDs. Here the NH is 5% that we have identified a low mass PMS population. The of the ONC. The younger stars in this cluster are at goals of this poster are to understand the X-ray lumi- their final masses. Stars are still relatively close to for- nosity function among the low mass PMS population mation spatially and are highly coeval. and to determine if this is a young star forming region Our optical photometric and spectroscopic follow- from the low mass population. up observations reveal ~250 low mass PMS stars with V<19, a space density of about 700 stars per square The X-ray observations will help us understand the degree [1]. Our latest deep photometry shows the PMS relation among the X-ray luminosity function, X-ray locus extends to I~19, well into the brown dwarf re- variability, disks and mass. These observations will gime [2]. We have spectroscopy of hundreds of PMS further our ultimate goal: understanding the nature of stars in the region. Our work has culminated in an un- magnetic activity for very low mass stars and BDs. biased optically selected sample of low mass stars and BDs. This, together with the low , makes Current Results: On the Chandra HRC observa- Sigma Ori the indisputably best place to look at X-ray tion, we detected over 140 X-ray sources in the field. emission from young BDs and TTs. We cross-correlated the positions with our optical catalogs and the 2MASS catalog. Since we don't have The 2 Myr age places it uniquely between the very complete correlations with all catalogs, we created two young associations (< 1Myr), and the older associa- different color magnitude diagrams to display our data. tions (e.g. α Per, Pleiades, H and Χ Persei, etc.; > 15 Myr). At younger ages (ONC, rho Oph), there is Figure 1 displays the optical sources (Naylor and highly variable extinction. There is almost no extinc- Jeffries I<14, Sherry et al., I>14) and the X-ray source tion here; this is critical due to the expected soft X- positions marked. We created an I vs. R-I color- rays from very low mass stars. This cluster is young magnitude diagram. Using I < 16 or R-I color < 1.5 enough that few stars have been lost to evaporation criteria, we limited our X-ray sources to candidate and at the stellar/sub-stellar boundary are PMS. These candidates are marked as red asterisks in still bright enough for X-ray detection. By just 5 Myr, Protostars and Planets V 2005 8425.pdf

figure 1. Other X-ray sources are marked as blue aster- isks.

Figure 2: 2MASS Color Diagram. Our Chandra X-ray detections were cross-correlated with the 2MASS catalog of this region. Mean extinction is A <1. Figure 1: Optical color-magnitude diagram for the sigma Orionis cluster. We cross-correlated the Chandra HRC X-ray source posi- tions and plotted the candidate PMS sources in red and other X-ray References source in blue. [1] Walter, F.M, Wolk, S.J., and Sherry, W.H.(1998) in Cool Stars 10. [2] Walter, F. M. and Wolk,

S.J.(2004) in Very Low Mass Stars and Brown Dwarfs An infrared color magnitude diagram was created in Cluster Associations. [3] Sherry W.H., Walter, F.M. with the 2MASS colors. Figure 2 displays the 2MASS and Wolk, S.J. AJ, 128, 2316 (2005). [4] Naylor, and sources (dots) and our X-ray detections; blue and red Jeffries, (2005) private communication. crosses. The dashed lines are the reddening vectors. The mean extinction is Av<1. There are 5 X-ray sources that show an IR excess, possibly optically thick disks.

We also created an X-ray luminosity diagram, however, this figure will be updated once we have the X-ray spectra for each source from the XMM-Newton data. We have identified 49 PMS correlations between the HRC observation and the spectra.

Acknowledgments: This research has made use of the NASA/IPAC Infra- red Science Archive and the Two Micron All Sky Sur- vey, which is a joint project of the University of Mas- sachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

This research is supported by NASA and the CXC through grant #GO2-3019A.