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Register a new userA Chandra and Spitzer census of the young star cluster in the reflection nebula NGC 7129
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B. Stelzer SUPA
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The reflection nebula NGC 7129 has long been known to be a site of recent star formation as evidenced, e.g., by the presence of deeply embedded protostars and HH objects. However, studies of the stellar population produced in the star formation process have remained rudimentary. At a presumed age of ~3 Myr, NGC7129 is in the critical range where disks around young stars disappear. We make use of Chandra X-ray and Spitzer and 2MASS IR imaging observations to identify the pre-main sequence stars in NGC7129. We define a sample of Young Stellar Objects based on color-color diagrams composed from IR photometry between 1.6 and 8 mu, from 2MASS and Spitzer, and based on X-ray detected sources from a Chandra observation. This sample is composed of 26 Class II and 25 Class III candidates. The sample is estimated to be complete down to ~ 0.5 solar masses. The most restricted and least biased sub-sample of pre-main sequence stars is composed of lightly absorbed (A_V < 5 mag) stars in the cluster core. This sample comprises 7 Class II and 14 Class III sources, it has a disk fraction of 33^{+24}_{-19} %, and a median X-ray luminosity of log (L_x) [erg/s] = 30.3. Despite the various uncertainties related to the sample selection, absorption, mass distribution, distance and, consequently, the computation of disk fraction and X-ray luminosities, the data yield consistent results. In particular, we confirm the age of ~3 Myr for the NGC7129 cluster. The derived disk fraction is similar to that of sigma Orionis, smaller than that of Cha I (~2 Myr), and larger than that of Upper Sco (5 Myr). The X-ray luminosity function is similar to that of NGC 2264 (2 Myr) but fainter than that of the Orion Nebula Cluster (1 Myr).
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The Pipe Nebula, a large nearby molecular cloud lacks obvious signposts of star formation in all but one of more than 130 dust extinction cores that have been identified within it. In order to quantitatively determine the current level of star formation activity in the Pipe Nebula, we analyzed 13 square degrees of sensitive mid-infrared maps of the entire cloud, obtained with the Multiband Imaging Photometer for Spitzer (MIPS) at wavelengths of 24 micron and 70 micron to search for candidate Young Stellar Objects (YSOs) in the high-extinction regions. We argue that our search is complete for class I and typical class II YSOs with luminosities of Lbol~0.2 Lo and greater. We find only 18 candidate YSOs in the high-extinction regions of the entire Pipe cloud. Twelve of these sources are previously known members of a small cluster associated with Barnard 59, the largest and most massive dense core in the cloud. With only six candidate class I and class II YSOs detected towards extinction cores outside of this cluster, our findings emphatically confirm the notion of an extremely low level of star formation activity in the Pipe Nebula. The resulting star formation efficiency for the entire cloud mass is only ~0.06 %.
NGC 7129 is a bright reflection nebula located in the molecular cloud complex near l=105.4, b=+9.9, about 1.15 kpc distant. Embedded within the reflection nebula is a young cluster dominated by a compact grouping of four early-type stars: BD+65 1638 (B3V), BD+65 1637 (B3e), SVS 13 (B5e), and LkH-alpha 234 (B8e). About 80 H-alpha emission sources brighter than V~23 are identified in the region, many of which are presumably T Tauri star members of the cluster. We also present deep (V~23), optical (VRI) photometry of a field centered on the reflection nebula and spectral types for more than 130 sources determined from low dispersion, optical spectroscopy. The narrow pre-main sequence evident in the color-magnitude diagram suggests that star formation was rapid and coeval. A median age of about 1.8 Myr is inferred for the H-alpha and literature-identified X-ray emission sources having established spectral types, using pre-main sequence evolutionary models. Our interpretation of the structure of the molecular cloud and the distribution of young stellar objects is that BD+65 1638 is primarily responsible for evacuating the blister-like cavity within the molecular cloud. LkH-alpha 234 and several embedded sources evident in near infrared adaptive optics imaging have formed recently within the ridge of compressed molecular gas. The compact cluster of low-mass stars formed concurrently with the early-type members, concentrated within a central radius of ~0.7 pc. Star formation is simultaneously occurring in a semi-circular arc some ~3 pc in radius that outlines remaining dense regions of molecular gas. High dispersion, optical spectra are presented for BD+65 1638, BD+65 1637, SVS 13, LkH-alpha 234, and V350 Cep. These spectra are discussed in the context of the circumstellar environments inferred for these stars.
We present results of a sensitive Chandra X-ray observation and Spitzer mid-IR observations of the infrared cluster lying north of the NGC 2071 reflection nebula in the Orion B molecular cloud. We focus on the dense cluster core known as NGC 2071-IR which contains at least nine IR sources within a 40 x 40 arcsecond region. This region shows clear signs of active star formation including powerful molecular outflows, Herbig-Haro objects, and both OH and H2O masers. We use Spitzer IRAC images to aid in X-ray source identification and to determine YSO classes using mid-IR colors. Spitzer IRAC colors show that the luminous source IRS 1 is a class I protostar. IRS 1 is believed to be driving a powerful bipolar molecular outflow and may be an embedded B-type star or its progenitor. Its X-ray spectrum reveals a fluorescent Fe emission line at 6.4 keV, arising in cold material near the protostar. The line is present even in the absence of large flares, raising questions about the nature of the ionizing mechanism responsible for producing the 6.4 keV fluorescent line. Chandra also detects X-ray sources at or near the positions of IRS 2, IRS 3, IRS 4, and IRS 6 and a variable X-ray source coincident with the radio source VLA 1, located just 2 arcsec north of IRS 1. No IR data are yet available to determine a YSO classification for VLA 1, but its high X-ray absorption shows that it is even more deeply-embedded than IRS 1, suggesting that it could be an even younger, less-evolved protostar.
We have performed mid-IR photometry of the young open cluster NGC 2264 using the images obtained with the Spitzer Space Telescope IRAC and MIPS instruments and present a normalized classification scheme of young stellar objects in various color-color diagrams to make full use of the information from multicolor photometry. These results are compared with the classification scheme based on the slope of the spectral energy distribution (SED). From the spatial distributions of Class I and II stars, we have identified two subclusterings of Class I objects in the CONE region of Sung et al. The disked stars in the other star forming region S MON are mostly Class II objects. These three regions show a distinct difference in the fractional distribution of SED slopes as well as the mean value of SED slopes. The fraction of stars with primordial disks is nearly flat between log m = 0.2 -- -0.5, and that of transition disks is very high for solar mass stars. In addition, we have derived a somewhat higher value of the primordial disk fraction for NGC 2264 members located below the main pre-main sequence locus (so-called BMS stars). This result supports the idea that BMS stars are young stars with nearly edge-on disks. We have also found that the fraction of primordial disks is very low near the most massive star S Mon and increases with distance from S Mon.
We present observations of a massive star cluster near the nuclear region of the nearby starburst galaxy NGC 253. The peak of near-infrared emission, which is spatially separated by 4 from the kinematic center of the galaxy, is coincident with a super star cluster whose properties we examine with low-resolution (R ~ 1,200) infrared CTIO spectroscopy and optical/near-infrared HST imaging. Extinction, measured from [FeII] lines, is estimated at Av = 17.7 +/- 2.6. The age of the cluster is estimated at 5.7 Myr, based on Bry equivalent width for an instantaneous burst using Starburst99 modeling. However, a complex star formation history is inferred from the presence of both recombination emission and photospheric CO absorption. The ionizing photon flux has a lower limit of 7.3 +/- 2.5 x 10^53 inverse seconds, corrected for extinction. Assuming a Kroupa IMF, we estimate a cluster mass of 1.4 +/- 0.4 x 10^7 solar masses. We observe a strong Wolf-Rayet signature at 2.06 microns and report a weak feature at 2.19 microns which may be due to a massive stellar population, consistent with the derived mass and age of this cluster.
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