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A Chandra and Spitzer census of the young star cluster in the reflection nebula NGC 7129
Context: The reflection nebula NGC 7129 has long been known to be a siteof recent star formation as shown, e.g., by the presence of deeplyembedded protostars and HH objects. However, studies of the stellarpopulation produced in the star formation process have remainedrudimentary. A major step forward has been made with recent Spitzerimaging of the region. Aims: This study represents the next steptowards a systematic assessment of the pre-main sequence population inNGC 7129. Completeness of the pre-main sequence sample is necessary forstudying key features that allow the star-forming process to beunderstood, such as disk evolution, dynamical evolution, and massfunction. At a presumed age of 3 Myr, NGC 7129 is in the critical rangewhere disks around young stars disappear. Methods: We make useof X-ray and IR imaging observations to identify the pre-main sequencestars in NGC 7129. We define a sample of young stellar objects based oncolor-color diagrams composed from IR photometry between 1.6 and 8?m, from 2 MASS and Spitzer, and based on X-ray detected sources froma Chandra observation. Results: This sample is composed of 26Class II and 25 Class III candidates. It has been selected from infraredsources in the Chandra field (287 objects with photometry in all fourSpitzer/IRAC bands, 811 objects with near-IR photometry) and the 59X-ray sources detected with Chandra. The sample is estimated to becomplete down to 0.5 {M_&sun;}. The most restricted and least biasedsubsample of pre-main sequence stars is composed of lightly absorbed(AV < 5 mag) stars in the cluster core. This samplecomprises 7 Class II and 14 Class III sources and has a disk fraction of33^+24-19% and a median X-ray luminosity of log{L_x} [erg/s]= 30.3. Conclusions: Despite the various uncertainties relatedto the sample selection, absorption, mass distribution, distance, and,consequently in 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 NGC 7129 cluster. The derived disk fraction issimilar to that of ? Orionis, smaller than for Cha I (˜ 2Myr), and larger than for Upper Sco (5 Myr). The X-ray luminosityfunction is similar to that of NGC 2264 (2 Myr) but fainter than for theOrion Nebula Cluster (1 Myr). The pre-main sequence census should befurther refined and extended with optical photometric and spectroscopicsearches for cluster members.Tables 2-6 are only available in electronic form at http://www.aanda.org
| Young Stars far from the Galactic Plane: Runaways from Clusters
Quite recently, a significant number of OB stars far from the galacticplane have been found, situated at z-distances ranging from severalhundreds of pc to several kpc. The short lifetimes of these stars poseproblems for their interpretation in terms of the standard picture ofstar formation. Different mechanisms have been put forward to explainthe existence of these stars, either within the conventional view, orpostulating star formation in the galactic halo itself. These mechanismsrange from arguing that they are misidentified evolved or abnormalstars, to postulating powerful ejection mechanisms for young disk stars;in situ formation also admits several variants. We have collected fromthe literature a list of young stars far from the plane, for which theevidence of youth seems convincing. We discuss two possible formationmechanisms for these stars: ejection from the plane as the result ofdynamical evolution of small clusters (Poveda et al. 1967) and in situformation, via induced shocks created by spiral density waves (Martos etal. 1999). We compute galactic orbits for these stars, and identify thestars that could be explained by one or the other mechanism. We findthat about 90 percent of the stars can be accounted for by the clusterejection mechanism, that is, they can be regarded as runaway stars inthe galactic halo.
| CCD images of suspected Herbig-Haro objects
Charge-coupled-device images in V, R, I, and H-alpha are presented forten regions suspected to contain Herbig-Haro objects, including 11 ofthe 37 HH candidates listed by Gyulbudaghian et al. (1978). Four of the11 are HH objects; the rest are either nebulous stars, reflectionnebulas, or plate flaws. The images of R CrA, Cep A, and NGC 7129 shedconsiderable light on the details of star formation in these regions.More than a dozen new HH objects have been identified in these regions;the data have been used to investigate the nature of energetic outflowactivity around these three objects.
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