- ID:
- ivo://CDS.VizieR/J/ApJ/762/128
- Title:
- Spitzer/IRS disk parameters in Serpens
- Short Name:
- J/ApJ/762/128
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Spectral energy distributions are presented for 94 young stars surrounded by disks in the Serpens Molecular Cloud, based on photometry and Spitzer/IRS spectra. Most of the stars have spectroscopically determined spectral types. Taking a distance to the cloud of 415pc rather than 259pc, the distribution of ages is shifted to lower values, in the 1-3Myr range, with a tail up to 10Myr. The mass distribution spans 0.2-1.2M_{sun}_, with median mass of 0.7M_{sun}_. The distribution of fractional disk luminosities in Serpens resembles that of the young Taurus Molecular Cloud, with most disks consistent with optically thick, passively irradiated disks in a variety of disk geometries (L_disk_/L_star_~0.1). In contrast, the distributions for the older Upper Scorpius and {eta} Chamaeleontis clusters are dominated by optically thin lower luminosity disks (L_disk_/L_star_~0.02). This evolution in fractional disk luminosities is concurrent with that of disk fractions: with time disks become fainter and the disk fractions decrease. The actively accreting and non-accreting stars (based on H{alpha} data) in Serpens show very similar distributions in fractional disk luminosities, differing only in the brighter tail dominated by strongly accreting stars. In contrast with a sample of Herbig Ae/Be stars, the T Tauri stars in Serpens do not have a clear separation in fractional disk luminosities for different disk geometries: both flared and flat disks present wider, overlapping distributions. This result is consistent with previous suggestions of a faster evolution for disks around Herbig Ae/Be stars. Furthermore, the results for the mineralogy of the dust in the disk surface (grain sizes, temperatures and crystallinity fractions, as derived from Spitzer/IRS spectra) do not show any correlation to either stellar and disk characteristics or mean cluster age in the 1-10Myr range probed here. A possible explanation for the lack of correlation is that the processes affecting the dust within disks have short timescales, happening repeatedly, making it difficult to distinguish long-lasting evolutionary effects.
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- ID:
- ivo://CDS.VizieR/J/ApJ/736/133
- Title:
- Spitzer-IRS study of massive YSOs in galactic center
- Short Name:
- J/ApJ/736/133
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from our spectroscopic study, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, designed to identify massive young stellar objects (YSOs) in the Galactic center (GC). Our sample of 107 YSO candidates was selected based on Infrared Array Camera (IRAC) colors from the high spatial resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone, which spans the central ~300pc region of the Milky Way. We obtained IRS spectra over 5-35um using both high- and low-resolution IRS modules. We spectroscopically identify massive YSOs by the presence of a 15.4um shoulder on the absorption profile of 15um CO_2_ ice, suggestive of CO_2_ ice mixed with CH_3_OH ice on grains. This 15.4um shoulder is clearly observed in 16 sources and possibly observed in an additional 19 sources. We show that nine massive YSOs also reveal molecular gas-phase absorption from CO_2_, C_2_H_2_, and/or HCN, which traces warm and dense gas in YSOs. Our results provide the first spectroscopic census of the massive YSO population in the GC. We fit YSO models to the observed spectral energy distributions and find YSO masses of 8-23M_{sun}_, which generally agree with the masses derived from observed radio continuum emission. We find that about 50% of photometrically identified YSOs are confirmed with our spectroscopic study. This implies a preliminary star formation rate of ~0.07M_{sun}/yr at the GC.
- ID:
- ivo://CDS.VizieR/J/ApJS/226/8
- Title:
- Spitzer/IRS survey of Class II objects in Orion A. I.
- Short Name:
- J/ApJS/226/8
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present our investigation of 319 Class II objects in Orion A observed by Spitzer/IRS. We also present the follow-up observations of 120 of these Class II objects in Orion A from the Infrared Telescope Facility/SpeX. We measure continuum spectral indices, equivalent widths, and integrated fluxes that pertain to disk structure and dust composition from IRS spectra of Class II objects in Orion A. We estimate mass accretion rates using hydrogen recombination lines in the SpeX spectra of our targets. Utilizing these properties, we compare the distributions of the disk and dust properties of Orion A disks with those of Taurus disks with respect to position within Orion A (Orion Nebular Cluster [ONC] and L1641) and with the subgroups by the inferred radial structures, such as transitional disks (TDs) versus radially continuous full disks (FDs). Our main findings are as follows. (1) Inner disks evolve faster than the outer disks. (2) The mass accretion rates of TDs and those of radially continuous FDs are statistically significantly displaced from each other. The median mass accretion rate of radially continuous disks in the ONC and L1641 is not very different from that in Taurus. (3) Less grain processing has occurred in the disks in the ONC compared to those in Taurus, based on analysis of the shape index of the 10{mu}m silicate feature (F_11.3_/F_9.8_). (4) The 20-31{mu}m continuum spectral index tracks the projected distance from the most luminous Trapezium star, {theta}^1^ Ori C. A possible explanation is UV ablation of the outer parts of disks.
- ID:
- ivo://CDS.VizieR/J/AJ/145/66
- Title:
- Spitzer light curves of YSOs in IC 348
- Short Name:
- J/AJ/145/66
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on synoptic observations at 3.6 and 4.5{mu}m of young stellar objects in IC 348 with 38epochs covering 40days. We find that among the detected cluster members, 338 at [3.6] and 269 at both [3.6] and [4.5], many are variable on daily to weekly timescales with typical fluctuations of ~0.1mag. The fraction of variables ranges from 20% for the diskless pre-main sequence stars to 60% for the stars still surrounded by infalling envelopes. We also find that stars in the exposed cluster core are less variable than the stars in the dense, slightly younger, southwestern ridge. This trend persists even after accounting for the underlying correlation with infrared spectral energy distribution type, suggesting that the change in variable fraction is not simply a reflection of the change in relative fraction of class I versus class II sources across the cloud, but instead reflects a change in variability with age. We also see a strong correlation between infrared variability and X-ray luminosity among the class II sources. The observed variability most likely reflects large changes in the structure of the inner wall located at the dust sublimation radius. We explore the possibility that these structural perturbations could be caused by a hot spot on the star heating dust above the sublimation temperature, causing it to evaporate rapidly, and increasing the inner radius for a portion of the disk. Under a number of simplifying assumptions we show that this model can reproduce the size and timescale of the 3.6 and 4.5{mu}m fluctuations. Regardless of its source, the infrared variability indicates that the inner disk is not a slowly evolving entity, but instead is a bubbling, warped, dented mass of gas and dust whose global size and shape fluctuate in a matter of days.
- ID:
- ivo://CDS.VizieR/J/AJ/144/31
- Title:
- Spitzer+2MASS photometry of protostar candidates
- Short Name:
- J/AJ/144/31
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We identify protostars in Spitzer surveys of nine star-forming (SF) molecular clouds within 1kpc: Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus, Orion, Cep OB3, and Mon R2, which combined host over 700 protostar candidates. These clouds encompass a variety of SF environments, including both low-mass and high-mass SF regions, as well as dense clusters and regions of sparsely distributed star formation. Our diverse cloud sample allows us to compare protostar luminosity functions in these varied environments. We combine near- and mid-infrared photometry from the Two Micron All Sky Survey and Spitzer to create 1-24{mu}m spectral energy distributions (SEDs). Using protostars from the c2d survey with well-determined bolometric luminosities, we derive a relationship between bolometric luminosity, mid-IR luminosity (integrated from 1-24{mu}m), and SED slope. Estimations of the bolometric luminosities for protostar candidates are combined to create luminosity functions for each cloud. Contamination due to edge-on disks, reddened Class II sources, and galaxies is estimated and removed from the luminosity functions. We find that luminosity functions for high-mass SF clouds (Orion, Mon R2, and Cep OB3) peak near 1L_{sun}_ and show a tail extending toward luminosities above 100L_{sun}_. The luminosity functions of the low-mass SF clouds (Serpens, Perseus, Ophiuchus, Taurus, Lupus, and Chamaeleon) do not exhibit a common peak, however the combined luminosity function of these regions peaks below 1L_{sun}_. Finally, we examine the luminosity functions as a function of the local surface density of young stellar objects. In the Orion molecular clouds, we find a significant difference between the luminosity functions of protostars in regions of high and low stellar density, the former of which is biased toward more luminous sources. This may be the result of primordial mass segregation, although this interpretation is not unique. We compare our luminosity functions to those predicted by models and find that our observed luminosity functions are best matched by models that invoke competitive accretion, although we do not find strong agreement between the high-mass SF clouds and any of the models.
- ID:
- ivo://CDS.VizieR/J/ApJ/682/445
- Title:
- Spitzer observations of S254-S258
- Short Name:
- J/ApJ/682/445
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present Spitzer IRAC, NOAO 2.1m Flamingos, Keck NIRC, and FCRAO SEQUOIA observations of the massive star-forming complex S254-S258, covering an area of 25'x20'. Using a combination of the IRAC and NIR data, we identify and classify the young stellar objects (YSOs) in the complex. We detect 510 sources with near- or mid-IR excess, and we classify 87 Class I and 165 Class II sources. The YSOs are found in clusters surrounded by isolated YSOs in a low-density distributed population. The ratio of clustered to total YSOs is 0.8. We identify six new clusters in the complex. One of them, G192.63-00, is located around the ionizing star of the HII region S255. We hypothesize that the ionizing star of S255 was formed in this cluster. We also detect a southern component of the cluster in HII region S256. The cluster G192.54-0.15, located inside HII region S254 has a V_LSR_ of 17km/s with respect to the main cloud, and we conclude that it is located in the background of the complex. The structure of the molecular cloud is examined using ^12^CO, and ^13^CO, as well as a near-IR extinction map. The main body of the molecular cloud has V_LSR_ between 5 and 9km/s. The arc-shaped structure of the molecular cloud following the border of the HII regions and the high column density in the border of the HII regions support the idea that the material has been swept up by the expansion of the HII regions.
- ID:
- ivo://CDS.VizieR/J/ApJS/186/111
- Title:
- Spitzer observations of Taurus members
- Short Name:
- J/ApJS/186/111
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have analyzed nearly all images of the Taurus star-forming region at 3.6, 4.5, 5.8, 8.0, and 24{mu}m that were obtained during the cryogenic mission of the Spitzer Space Telescope (46deg^2^) and have measured photometry for all known members of the region that are within these data, corresponding to 348 sources, or 99% of the known stellar population. By combining these measurements with previous observations with the Spitzer Infrared Spectrograph and other facilities, we have classified the members of Taurus according to whether they show evidence of circumstellar disks and envelopes (classes I, II, and III). Through these classifications, we find that the disk fraction in Taurus, N(II)/N(II+III), is ~75% for solar-mass stars and declines to ~45% for low-mass stars and brown dwarfs (0.01-0.3M_{sun}_). Through an analysis of multiple epochs of Spitzer photometry that are available for ~200 Taurus members, we find that stars with disks exhibit significantly greater mid-infrared (mid-IR) variability than diskless stars, which agrees with the results of similar variability measurements for a smaller sample of stars in Chamaeleon I. The variability fraction for stars with disks is higher in Taurus than in Chamaeleon I, indicating that the IR variability of disks decreases with age. Finally, we have used our data in Taurus to refine the observational criteria for primordial, evolved, and transitional disks. The ratio of the number of evolved and transitional disks to the number of primordial disks in Taurus is 15/98 for spectral types of K5-M5, indicating a timescale of 0.15x{tau}_primordial_~0.45Myr for the clearing of the inner regions of optically thick disks.
- ID:
- ivo://CDS.VizieR/J/ApJ/743/39
- Title:
- Spitzer observations of W3 molecular cloud
- Short Name:
- J/ApJ/743/39
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this work, we have carried out an in-depth analysis of the young stellar content in the W3 giant molecular cloud (GMC). The young stellar object (YSO) population was identified and classified in the Infrared Array Camera/Multiband Imaging Photometer color-magnitude space according to the "Class" scheme and compared to other classifications based on intrinsic properties. Class 0/I and II candidates were also compared to low-/intermediate-mass pre-main-sequence (PMS) stars selected through their colors and magnitudes in the Two Micron All Sky Survey. We find that a reliable color/magnitude selection of low-mass PMS stars in the infrared requires prior knowledge of the protostar population, while intermediate-mass objects can be more reliably identified. By means of the minimum spanning tree algorithm and our YSO spatial distribution and age maps, we investigated the YSO groups and the star formation history in W3.
- ID:
- ivo://CDS.VizieR/J/ApJ/880/9
- Title:
- Spitzer obs. of YSOs in the SMOG field
- Short Name:
- J/ApJ/880/9
- Date:
- 07 Mar 2022 07:09:50
- Publisher:
- CDS
- Description:
- In this paper we undertake a study of the 21deg^2^ SMOG field, a Spitzer cryogenic mission Legacy program to map a region of the outer Milky Way toward the Perseus and outer spiral arms with the IRAC and MIPS instruments. We identify 4648 YSOs across the field. Using the DBSCAN method, we identify 68 clusters or aggregations of YSOs in the region, having eight or more members. We identify 1197 Class I objects, 2632 Class II objects, and 819 Class III objects, of which 45 are candidate transition disk objects, utilizing the MIPS 24 photometry. The ratio of YSOs identified as members of clusters was 2872/4648, or 62%. The ratios of Class I to Class II YSOs in the clusters are broadly consistent with those found in the inner Galactic and nearby Gould Belt young star formation regions. The clustering properties indicate that the protostars may be more tightly bound to their natal sites than the Class II YSOs, and the Class III YSOs are generally widely distributed. We further perform an analysis of the WISE data of the SMOG field to determine how the lower resolution and sensitivity of WISE affects the identification of YSOs as compared to Spitzer: we identify 931 YSOs using combined WISE and 2MASS photometry, or 20% (931/4648) of the total number identified with Spitzer. Performing the same clustering analysis finds 31 clusters that reliably trace the larger associations identified with the Spitzer data. Twelve of the clusters identified have previously measured distances from the WISE HII survey. SEDFitter modeling of these YSOs is reported, leading to an estimation of the initial mass function in the aggregate of these clusters that approximates that found in the inner Galaxy, implying that the processes behind stellar mass distribution during star formation are not widely affected by the lower density and metallicity of the outer Galaxy.
- ID:
- ivo://CDS.VizieR/J/AJ/144/192
- Title:
- Spitzer survey of Orion A and B. I. YSO catalog
- Short Name:
- J/AJ/144/192
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a survey of the Orion A and B molecular clouds undertaken with the IRAC and MIPS instruments on board Spitzer. In total, five distinct fields were mapped, covering 9deg^2^ in five mid-IR bands spanning 3-24{mu}m. The survey includes the Orion Nebula Cluster, the Lynds 1641, 1630, and 1622 dark clouds, and the NGC 2023, 2024, 2068, and 2071 nebulae. These data are merged with the Two Micron All Sky Survey point source catalog to generate a catalog of eight-band photometry. We identify 3479 dusty young stellar objects (YSOs) in the Orion molecular clouds by searching for point sources with mid-IR colors indicative of reprocessed light from dusty disks or infalling envelopes. The YSOs are subsequently classified on the basis of their mid-IR colors and their spatial distributions are presented. We classify 2991 of the YSOs as pre-main-sequence stars with disks and 488 as likely protostars. Most of the sources were observed with IRAC in two to three epochs over six months; we search for variability between the epochs by looking for correlated variability in the 3.6 and 4.5{mu}m bands. We find that 50% of the dusty YSOs show variability. The variations are typically small (~0.2mag) with the protostars showing a higher incidence of variability and larger variations. The observed correlations between the 3.6, 4.5, 5.8, and 8{mu}m variability suggests that we are observing variations in the heating of the inner disk due to changes in the accretion luminosity or rotating accretion hot spots.
