arXiv subjects

Marc Kassis

Publications and source records attributed to Marc Kassis.

27 records · Page 2Linked to original sources

arXiv2015

The MOSFIRE Deep Evolution Field (MOSDEF) Survey: Rest-Frame Optical Spectroscopy for ~1500 H-Selected Galaxies at 1.37 < z < 3.8

In this paper we present the MOSFIRE Deep Evolution Field (MOSDEF) survey. The MOSDEF survey aims to obtain moderate-resolution (R=3000-3650) rest-frame optical spectra (~3700-7000 Angstrom) for ~1500 galaxies at 1.37<z<3.80 in three well-studied CANDELS fields: AEGIS, COSMOS, and GOODS-N. Targets are selected in three redshift intervals: 1.37<z<1.70, 2.09<z<2.61, and 2.95<z<3.80, down to fixed H_AB (F160W) magnitudes of 24.0, 24.5 and 25.0, respectively, using the photometric and spectroscopic catalogs from the 3D-HST survey. We target both strong nebular emission lines (e.g., [OII], Hbeta, [OIII], 5008, Halpha, [NII], and [SII]) and stellar continuum and absorption features (e.g., Balmer lines, Ca-II H and K, Mgb, 4000 Angstrom break). Here we present an overview of our survey, the observational strategy, the data reduction and analysis, and the sample characteristics based on spectra obtained during the first 24 nights. To date, we have completed 21 masks, obtaining spectra for 591 galaxies. For ~80% of the targets we derive a robust redshift from either emission or absorption lines. In addition, we confirm 55 additional galaxies, which were serendipitously detected. The MOSDEF galaxy sample includes unobscured star-forming, dusty star-forming, and quiescent galaxies and spans a wide range in stellar mass (~10^9-10^11.5 Msol) and star formation rate (~10^0-10^3 Msol/yr). The spectroscopically confirmed sample is roughly representative of an H-band limited galaxy sample at these redshifts. With its large sample size, broad diversity in galaxy properties, and wealth of available ancillary data, MOSDEF will transform our understanding of the stellar, gaseous, metal, dust, and black hole content of galaxies during the time when the universe was most active.

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arXiv2014

Distributed Low-Mass Star Formation in the IRDC G34.43+00.24

We have used deep near-infrared observations with adaptive optics to discover a distributed population of low-mass protostars within the filamentary Infrared Dark Cloud G34.43+00.24. We use maps of dust emission at multiple wavelengths to determine the column density structure of the cloud. In combination with an empirically-verified model of the magnitude distribution of background stars, this column density map allows us to reliably determine overdensities of red sources that are due to embedded protostars in the cloud. We also identify protostars through their extended emission in K-band which comes from excited H2 in protostellar outflows or reflection nebulosity. We find a population of distributed low-mass protostars, suggesting that low-mass protostars may form earlier than, or contemporaneously with, high-mass protostars in such a filament. The low-mass protostellar population may also produce the narrow linewidth SiO emission observed in some clouds without high-mass protostars. Finally, we use a molecular line map of the cloud to determine the virial parameter per unit length along the filament and find that the highest mass protostars form in the most bound portion of the filament, as suggested by theoretical models.

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arXiv2013

The curious morphology and orientation of Orion proplyd HST-10

HST-10 is one of the largest proplyds in the Orion Nebula and is located approximately 1' SE of the Trapezium. Unlike other proplyds in Orion, however, the long-axis of HST-10 does not align with theta 1 C Ori, but is instead aligned with the rotational axis of the HST-10 disk. This cannot be easily explained using current photo-evaporation models. In this letter, we present high spatial resolution near-infrared images of the Orion proplyd HST-10 using Keck/NIRC2 with the Laser Guide Star Adaptive Optics system, along with multi-epoch analysis of HH objects near HST-10 using Hubble Space Telescope WFPC2 and ACS cameras. Our narrow-band near-IR images resolve the proplyd ionization front (IF) and circumstellar disk down to 23 AU at the distance to Orion in Br gamma, He I, H_2, and PAH emission. Br gamma and He I emission primarily trace the IF (with the disk showing prominently in silhouette), while the H_2 and PAH emission trace the surface of the disk itself. PAH emission also traces small dust grains within the proplyd envelope which is asymmetric and does not coincide with the IF. The curious morphology of the PAH emission may be due to UV-heating by both theta 1C Ori and theta 2A Ori. Multi-epoch HST images of the HST-10 field show proper motion of 3 knots associated with HH 517, clearly indicating that HST-10 has a jet. We postulate that the orientation of HST-10 is determined by the combined ram-pressure of this jet and the FUV-powered photo-ablation flow from the disk surface.

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arXiv2013

ALMA Observations of the IRDC Clump G34.43+00.24 MM3: Hot Core and Molecular Outflows

We have observed a cluster forming clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the 1.3 mm continuum and the CH3OH, CS, 13CS, SiO, CH3CH2CN, and HCOOCH3 lines with the Atacama Large Millimeter/submillimeter Array and in K-band with the Keck telescope. We have found a young outflow toward the center of this clump in the SiO, CS, and CH3OH lines. This outflow is likely driven by a protostar embedded in a hot core, which is traced by the CH3CH2CN, HCOOCH3, 13CS, and high excitation CH3OH lines. The size of the hot core is about 800 x 300 AU in spite of its low mass (<1.1 M_sun), suggesting a high accretion rate or the presence of multiple star system harboring a few hot corinos. The outflow is highly collimated, and the dynamical timescale is estimated to be less than 740 yr. In addition, we have also detected extended emission of SiO, CS, and CH3OH, which is not associated with the hot core and the outflow. This emission may be related to past star formation activity in the clump. Although G34.43+00.24 MM3 is surrounded by a dark feature in infrared, it has already experienced active formation of low-mass stars in an early stage of clump evolution.

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arXiv2012

Spectral classification of the brightest objects in the galactic star forming region W40

We present high S/N, moderate resolution near-infrared spectra, as well as 10 micron imaging, for the brightest members of the central stellar cluster in the W40 HII region, obtained using the SpeX and MIRSI instruments at NASA's Infrared Telescope Facility. Using these observations combined with archival Spitzer Space Telescope data, we have determined the spectral classifications, extinction, distances, and spectral energy distributions for the brightest members of the cluster. Of the eight objects observed, we identify four main sequence (MS) OB stars, two Herbig Ae/Be stars, and two low-mass young stellar objects. Strong HeI absorption at 1.083 micron in the MS star spectra strongly suggests that at least some of these sources are in fact close binaries. Two out of the four MS stars also show significant infrared excesses typical of circumstellar disks. Extinctions and distances were determined for each MS star by fitting model stellar atmospheres to the SEDs. We estimate a distance to the cluster of between 455 and 535 pc, which agrees well with earlier (but far less precise) distance estimates. We conclude that the late-O star we identify is the dominant source of LyC luminosity needed to power the W40 HII region and is the likely source of the stellar wind that has blown a large (~4 pc) pinched-waist bubble observed in wide field mid-IR images. We also suggest that 3.6 cm radio emission observed from some of the sources in the cluster is likely not due to emission from ultra-compact HII regions, as suggested in other work, due to size constraints based on our derived distance to the cluster. Finally, we also present a discussion of the curious source IRS 3A, which has a very strong mid-IR excess (despite its B3 MS classification) and appears to be embedded in a dusty envelope roughly 2700 AU in size.

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arXiv2011

Episodic mass loss in binary evolution to the Wolf-Rayet phase: Keck and HST proper motions of RY Scuti's nebula

Binary mass transfer via Roche-lobe overflow (RLOF) is a key channel for producing stripped-envelope Wolf-Rayet (WR) stars and may be critical to account for SN Ib/c progenitors. RY Scuti is an extremely rare example of a massive binary star caught in this brief but important phase. Its toroidal nebula indicates equatorial mass loss during RLOF, while the mass-gaining star is apparently embedded in an opaque accretion disk. RY Scuti's toroidal nebula has two components: an inner ionised double-ring system, and an outer dust torus that is twice the size of the ionised rings. We present two epochs of Lband Keck NGS-AO images of the dust torus, plus three epochs of HST images of the ionised gas rings. Proper motions show that the inner ionised rings and the outer dust torus came from two separate ejection events roughly 130 and 250 yr ago. This suggests that RLOF in massive contact binaries can be accompanied by eruptive and episodic burst of mass loss, reminiscent of LBVs. We speculate that the repeating outbursts may arise in the mass gainer from instabilities associated with a high accretion rate. If discrete mass-loss episodes in other RLOF binaries are accompanied by luminous outbursts, they might contribute to the population of extragalactic optical transients. When RLOF ends for RY Scuti, the overluminous mass gainer, currently surrounded by an accretion disk, will probably become a B[e] supergiant and may outshine the hotter mass-donor star that should die as a Type Ib/c supernova.

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arXiv2009

Diverse protostellar evolutionary states in the young cluster AFGL961

We present arcsecond resolution mid-infrared and millimeter observations of the center of the young stellar cluster AFGL961 in the Rosette molecular cloud. Within 0.2 pc of each other, we find an early B star embedded in a dense core, a neighboring star of similar luminosity with no millimeter counterpart, a protostar that has cleared out a cavity in the circumcluster envelope, and two massive, dense cores with no infrared counterparts. An outflow emanates from one of these cores, indicating a deeply embedded protostar, but the other is starless, bound, and appears to be collapsing. The diversity of states implies either that protostellar evolution is faster in clusters than in isolation or that clusters form via quasi-static rather than dynamic collapse. The existence of a pre-stellar core at the cluster center shows that that some star formation continues after and in close proximity to massive, ionizing stars.

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arXiv2006

Silicate Emission Profiles from Low-Mass Protostellar Disks in the Orion Nebula: Evidence for Growth and Thermal Processing of Grains

We present 8--13 micron low resolution spectra (R~100) of 8 low-mass protostellar objects ("proplyds") in the Orion Nebula using the Long Wavelength Spectrometer (LWS) at the W. M. Keck Observatory. All but one of the sources in our sample show strong circumstellar silicate emission, with profiles that are qualitatively similar to those seen in some T Tauri and Herbig Ae/Be stars. The silicate profile in all cases is significantly flattened compared to the profile for typical interstellar dust, suggesting that the dominant emitting grains are significantly larger than those found in the interstellar medium. The 11.3-to-9.8 micron flux ratio--often used as an indicator of grain growth--is in the 0.8 to 1.0 range for all of our targets, indicating that the typical grain size is around a few microns in the surface layers of the attendant circumstellar disk for each object. Furthermore, the silicate profiles show some evidence of crystalline features, as seen in other young stellar objects. The results of our analysis show that the grains in the photoevaporating protostellar disks of Orion have undergone significant growth and perhaps some annealing, suggesting that grain evolution for these objects is not qualitatively different from other young stellar objects.

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arXiv2005

Thermal Dust Emission from Proplyds, Unresolved Disks, and Shocks in the Orion Nebula

We present a new 11.7 micron mosaic image of the Orion nebula obtained with T-ReCS on Gemini South. The map includes the BN/KL region, the Trapezium, and OMC-1 South. Excluding BN/KL, we detect 91 point sources, with 27 known proplyds and over 30 ``naked'' stars showing no extended structure in HST images. Within the region we surveyed, 80 percent of known proplyds show detectable emission, almost 40 percent of naked stars are detected at 11.7 micron, and the fraction of all visible sources with IR excess emission is roughly 50 percent. Thermal dust emission from stars with no extended structure in HST images means that they have dust disks comparable to the size of our solar system. Proplyds and stars with IR excess show a clear anti-correlation in their spatial distribution, with proplyds clustered close to theta1C, and other infrared sources found farther away. We suspect that the clustered proplyds trace the youngest 0.5 Myr age group associated with the Trapezium, while the more uniformly-distributed sources trace the older 1-2 Myr population of the ONC. This suggests that small disks persist for a few Myr in irradiated environments, and hints that hierarchical sub-clustering has been important. Within 30 arcsec of theta1C, all proplyds are detected at 11.7 micron. The star theta1D is associated with the most prominent mid-IR dust arc in the nebula. We propose that this arc is the consequence of theta1D being the closest member of the Trapezium to the background cloud. Finally, we detect dust emission from HH jets in Orion, including HH202, HH529, HH513, and HH514. This is the first detection of mid-IR continuum emission from dust in the body of a collimated HH jet or bow shock.

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