arXiv subjects

Angela S. Cotera

Publications and source records attributed to Angela S. Cotera.

16 recordsLinked to original sources

arXiv2026

The Sagittarius C Complex in the Mid-Infrared with SOFIA/FORCAST

We present an analysis of high-resolution mid-infrared observations at 25 and 37 $μm$ of the Sagittarius C Complex (Sgr C) in the Central Molecular Zone (CMZ), based on data from the SOFIA/FORCAST Galactic Center Legacy Survey. Enabled by the high bright-source limit of the FORCAST instrument, we perform a map-level dust temperature and optical depth analysis with a focus on the Sgr C HII region, which has an average dust temperature of 61 K and an average 37 $μm$ optical depth of 0.05. We find that the Sgr C HII region contains several high-density dust emission ridges, with lengths of up to several parsecs. Noting prior evidence for nonthermal radio emission from these density ridges, we postulate that there is an enhancement of relativistic electrons within them, possibly attributable to diffusive shock acceleration induced by the wind of a known nearby Wolf-Rayet (WR) star impacting the density ridges and the ambient gas in the surrounding photo-dissociation region. Additionally, the tangential magnetic field in the outskirts of the Sgr C HII region may serve to confine the electrons within this region. We examined the heating effect of the WR star by calculating its heating profile and performing a spectral energy distribution modelling of the HII region. We found an integrated MIR luminosity of $(1.40\pm0.19)\times10^{6} L_\odot$, which implies that presently unidentified massive stars must be present in the HII region in addition to the WR star. We also present a brief analysis of adjacent regions, such as a mid-infrared/radio source denoted "Source C" and the G359.43+0.02 young stellar object cluster near the northern end of the prominent Sgr C non-thermal filament (NTF).

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arXiv2024

SOFIA/FORCAST Galactic Center Source Catalog

The central regions of the Milky Way constitute a unique laboratory for a wide swath of astrophysical studies, consequently the inner $\sim$400 pc has been the target of numerous large surveys at all accessible wavelengths. In this paper we present a catalog of sources at 25 and 37 $μ$m located within all of the regions observed with the SOFIA/FORCAST instrument in the inner $\sim$200 pc of the Galaxy. The majority of the observations were obtained as part of the SOFIA Cycle 7 Galactic Center Legacy program survey, which was designed to complement the Spitzer/MIPS 24 $μ$m catalog in regions saturated in the MIPS observations. Due to the wide variety of source types captured by our observations at 25 and 37 $μ$m, we do not limit the FORCAST source catalog to unresolved point sources, or treat all sources as if they are point-like sources. The catalog includes all detectable sources in the regions, resulting in a catalog of 950 sources, including point sources, compact sources, and extended sources. We also provide the user with metrics to discriminate between the source types.

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arXiv2021

Hunting for Planetary Nebulae toward the Galactic Center

We present near-infrared (IR) spectra of two planetary nebula (PN) candidates in close lines of sight toward the Galactic center (GC) using the Gemini Near-Infrared Spectrograph (GNIRS) at Gemini North. High-resolution images from radio continuum and narrow-band IR observations reveal ringlike or barrel-shaped morphologies of these objects, and their mid-IR spectra from the Spitzer Space Telescope exhibit rich emission lines from highly-excited species such as [S IV], [Ne III], [Ne V], and [O IV]. We also derive elemental abundances using the Cloudy synthetic models, and find an excess amount of the $s$-process element Krypton in both targets, which supports their nature as PN. We estimate foreground extinction toward each object using near-IR hydrogen recombination lines, and find significant visual extinctions ($A_V > 20$). The distances inferred from the size versus surface brightness relation of other PNe are $9.0\pm1.6$ kpc and $7.6\pm1.6$ kpc for SSTGC 580183 and SSTGC 588220, respectively. These observed properties along with abundance patterns and their close proximity to Sgr A$^*$ (projected distances $<20$ pc) make it highly probable that these objects are the first confirmed PN objects in the nuclear stellar disk. The apparent scarcity of such objects resembles the extremely low rate of PN formation in old stellar systems, but is in line with the current rate of the sustained star formation activity in the Central Molecular Zone.

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arXiv2021

Sagittarius B1 -- A Patchwork of H II Regions and PhotoDissociation Regions

Sgr B1 is a luminous H II region in the Galactic Center immediately next to the massive star-forming giant molecular cloud Sgr B2 and apparently connected to it from their similar radial velocities. In 2018 we showed from SOFIA FIFI-LS observations of the [O III] 52 and 88 micron lines that there is no central exciting star cluster and that the ionizing stars must be widely spread throughout the region. Here we present SOFIA FIFI-LS observations of the [O I] 146 and [C II] 158 micron lines formed in the surrounding photodissociation regions (PDRs). We find that these lines correlate neither with each other nor with the [O III] lines although together they correlate better with the 70 micron Herschel PACS images from Hi-GAL. We infer from this that Sgr B1 consists of a number of smaller H II regions plus their associated PDRs, some seen face-on and the others seen more or less edge-on. We used the PDR Toolbox to estimate densities and the far-ultraviolet intensities exciting the PDRs. Using models computed with Cloudy, we demonstrate possible appearances of edge-on PDRs and show that the density difference between the PDR densities and the electron densities estimated from the [O III] line ratios is incompatible with pressure equilibrium unless there is a substantial pressure contribution from either turbulence or magnetic field or both. We likewise conclude that the hot stars exciting Sgr B1 are widely spaced throughout the region at substantial distances from the gas with no evidence of current massive star formation.

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arXiv2020

SOFIA/FORCAST Galactic Center Legacy Survey: Overview

The Galactic Center contains some of the most extreme conditions for star formation in our Galaxy as well as many other phenomena that are unique to this region. Given our relative proximity to the Galactic Center, we are able to study details of physical processes to a level that is simply not yet possible for more distant galaxies, yielding an otherwise inaccessible view of the nuclear region of a galaxy. We recently carried out a targeted imaging survey of mid-infrared bright portions of the Galactic Center at 25 and 37 $μ$m using the FORCAST instrument on SOFIA. This survey was one of the inaugural Legacy Programs from SOFIA cycle 7, observing a total area of 403 arcmin$^2$ (2180 pc$^2$), including the Sgr A, B, and C complexes. Here we present an overview of the survey strategy, observations, and data reduction as an accompaniment to the initial public release of the survey data. We discuss interesting regions and features within the data including extended features near the circumnuclear disk, structures in the Arched Filaments and Sickle H II regions, and signs of embedded star formation in Sgr B2 and Sgr C. We also feature a handful of less well studied mid-infrared sources located between Sgr A and Sgr C that could be sites of relatively isolated star formation activity. Last, we discuss plans for subsequent publications and future data releases from the survey.

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arXiv2018

SOFIA FIFI-LS Observations of Sgr B1: Ionization Structure and Sources of Excitation

The current paradigm of Galactic Center (GC) gas motions and star formation envisions sequential star formation in streams of gas as they pass near the supermassive black hole, Sgr A*. This is based on the relative positions of dense molecular clouds, the very young star-forming region Sgr B2, the much older region Sgr C, and the several Myr old Arches and Quintuplet Clusters. Because Sgr B1 is found with Sgr B2 in a common envelope of molecular gas and far-infrared emission, the two sources are thought to be physically related, even though there are indicators of a significantly greater age for Sgr B1. To clarify the status of Sgr B1, we have mapped it with the FIFI-LS spectrometer on SOFIA in the far-infrared lines of [O III] 52 and 88 micron. From the ratios of these lines and lines measured with the Spitzer Infrared Spectrograph, we find that there are at least eight separate sub-regions that must contain the stars that excite the gas. We infer spectral energy distributions (SEDs) of the ionizing sources from models and find they are in agreement only with SEDs of late O stars augmented at the highest frequencies with interstellar X-rays from fast shocks. We suggest that although the gas, from its velocity structure, must be part of the very young Sgr B2 complex, the stars that are ionizing the gas were not formed there but are the remnants of a previous generation of star formation in the GC.

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arXiv2013

Aligned grains and inferred toroidal magnetic fields in the envelopes of massive young stellar objects

Massive young stellar objects (YSOs), like low-mass YSOs, are thought to be surrounded by optically thick envelopes and/or discs and are observed to have associated regions that produce polarized light at near-infrared wavelengths. These polarized regions are thought to be lower-density outflows along the polar axes of the YSO envelopes. Using the 0.2 arcsec spatial resolution of the Near-Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope we are examining the structure of the envelopes and outflow regions of massive YSOs in star-forming regions within a few kpc of the Sun. Here we report on 2 micron polarimetry of Mon R2-IRS3, S140-IRS1, and AFGL 2591. All three sources contain YSOs with highly-polarized monopolar outflows, with Mon R2-IRS3 containing at least two YSOs in a small cluster. The central stars of all four YSOs are also polarized, with position angles perpendicular to the directions of the outflows. We infer that this polarization is due to scattering and absorption by aligned grains. We have modelled our observations of S140-IRS1 and AFGL 2591 as light scattered and absorbed both by spherical grains and by elongated grains that are aligned by magnetic fields. Models that best reproduce the observations have a substantial toroidal component to the magnetic field in the equatorial plane. Moreover, the toroidal magnetic field in the model that best fits AFGL 2591 extends a large fraction of the height of the model cavity, which is 10^5 au. We conclude that the massive YSOs in this study all show evidence of the presence of a substantial toroidal magnetic field.

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arXiv2011

A Spitzer Space Telescope survey of massive young stellar objects in the G333.2-0.4 giant molecular cloud

The G333 giant molecular cloud contains a few star clusters and H II regions, plus a number of condensations currently forming stars. We have mapped 13 of these sources with the appearance of young stellar objects (YSOs) with the Spitzer Infrared Spectrograph in the SL, SH, and LH modules (5-36 micron). We use these spectra plus available photometry and images to characterize the YSOs. The spectral energy distributions (SEDs) of all sources peak between 35 and 110 micron, thereby showing their young age. The objects are divided into two groups: YSOs associated with extended emission in IRAC band 2 at 4.5 micron (`outflow sources') and YSOs that have extended emission in all IRAC bands peaking at the longest wavelengths (`red sources'). The two groups of objects have distinctly different spectra: All the YSOs associated with outflows show evidence of massive envelopes surrounding the protostar because the spectra show deep silicate absorption features and absorption by ices at 6.0, 6.8, and 15.2 micron. We identify these YSOs with massive envelopes cool enough to contain ice-coated grains as the `bloated' protostars in the models of Hosokawa et al. All spectral maps show ionized forbidden lines and PAH emission features. For four of the red sources, these lines are concentrated to the centres of the maps, from which we infer that these YSOs are the source of ionizing photons. Both types of objects show evidence of shocks, with most of the outflow sources showing a line of [S I] in the outflows and two of the red sources showing the more highly excited [Ne III] and [S IV] lines in outflow regions at some distance from the YSOs. The 4.5 micron emission seen in the IRAC band 2 images of the outflow sources is not due to H2 lines, which are too faint in the 5-10 micron wavelength region to be as strong as is needed to account for the IRAC band 2 emission.

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arXiv2011

Massive Young Stellar Objects in the Galactic Center. I. Spectroscopic Identification from Spitzer/IRS Observations

We present results from our spectroscopic study, using the Infrared Spectrograph (IRS) onboard 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 IRAC colors from the high spatial resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone (CMZ), which spans the central ~300 pc region of the Milky Way Galaxy. We obtained IRS spectra over 5um to 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 CO2 ice, suggestive of CO2 ice mixed with CH3OH ice on grains. This 15.4um shoulder is clearly observed in 16 sources and possibly observed in an additional 19 sources. We show that 9 massive YSOs also reveal molecular gas-phase absorption from CO2, C2H2, 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 - 23 Msun, 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 ~0.07 Msun/yr at the GC.

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arXiv2010

Discovery of a Luminous Blue Variable with an Ejection Nebula Near the Quintuplet Cluster

We report on the discovery of a luminous blue variable (LBV) lying ~7 pc in projection from the Quintuplet cluster. This source, which we call LBV G0.120-0.048, was selected for spectroscopy owing to its detection as a strong source of Paschen-alpha excess in a recent narrowband imaging survey of the Galactic center region with HST/NICMOS. The K-band spectrum is similar to that of the Pistol Star and other known LBVs. The new LBV was previously cataloged as a photometric variable star, exhibiting brightness fluctuations of up to ~1 mag between 1994 and 1997, with significant variability also occurring on month-to-month time scales. The luminosity of LBV G0.120-0.048, as derived from 2MASS photometry, is approximately equivalent to that of the Pistol Star. However, the time-averaged brightness of LBV G0.120-0.048 between 1994 and 1997 exceeded that of the Pistol Star; LBV G0.120-0.048 also suffers more extinction, which suggests that it was intrinsically more luminous in the infrared than the Pistol Star between 1994 and 1997. Paschen-alpha images reveal a thin circular nebula centered on LBV G0.120-0.048 with a physical radius of ~0.8 pc. We suggest that this nebula is a shell of ejected material launched from a discrete eruption that occurred between 5000 and 10,000 years ago. Because of the very short amount of time that evolved massive stars spend in the LBV phase, and the close proximity of LBV G0.120-0.048 to the Quintuplet cluster, we suggest that this object might be coeval with the cluster and may have once resided within it.

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arXiv2009

Near-infrared counterparts to Chandra X-ray sources toward the Galactic Center. II. Discovery of Wolf-Rayet stars and O supergiants

We present new identifications of infrared counterparts to the population of hard X-ray sources near the Galactic center detected by the Chandra X-ray Observatory. We have confirmed 16 new massive stellar counterparts to the X-ray population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars, and O supergiants. For the majority of these sources, the X-ray photometry is consistent with thermal emission from plasma having temperatures in the range of kT=1-8 keV or non-thermal emission having power-law indices in the range of -1<gamma<3, and X-ray luminosities in the range of Lx~1e32-1e34 erg/s. Several sources have exhibited X-ray variability of several factors between separate observations. The X-ray properties are not a ubiquitous feature of single massive stars but are typical of massive binaries, in which the high-energy emission is generated by the collision of supersonic winds, or by accretion onto a compact companion. However, the possibility of intrinsic hard X-ray generation from single stars cannot be completely ruled out. The spectral energy distributions of these sources exhibit significant infrared excess, attributable to free-free emission from ionized stellar winds, supplemented by hot dust emission in the case of the WC stars. With the exception of one object located near the outer regions of the Quintuplet cluster, most of the new stars appear isolated or in loose associations. Seven hydrogen-rich WN and O stars are concentrated near the Sagittarius B HII region, while other similar stars and more highly evolved hydrogen-poor WN and WC stars lie scattered within ~50 pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable of generating the observed X-rays and the implications these stars have for massive star formation in the Galaxy's Central Molecular Zone.

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arXiv2009

First Spectroscopic Identification of Massive Young Stellar Objects in the Galactic Center

We report the detection of several molecular gas-phase and ice absorption features in three photometrically-selected young stellar object (YSO) candidates in the central 280 pc of the Milky Way. Our spectra, obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, reveal gas-phase absorption from CO2 (15.0um), C2H2 (13.7um) and HCN (14.0um). We attribute this absorption to warm, dense gas in massive YSOs. We also detect strong and broad 15um CO2 ice absorption features, with a remarkable double-peaked structure. The prominent long-wavelength peak is due to CH3OH-rich ice grains, and is similar to those found in other known massive YSOs. Our IRS observations demonstrate the youth of these objects, and provide the first spectroscopic identification of massive YSOs in the Galactic Center.

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arXiv2009

Hubble Space Telescope NICMOS Polarization Observations of Three Edge-on Massive YSOs

Massive young stellar objects (YSOs), like low-mass YSOs, appear to be surrounded by optically thick envelopes and/or disks and have regions, often bipolar, that are seen in polarized scattered light at near-infrared wavelengths. We are using the 0.2'' spatial resolution of NICMOS on Hubble Space Telescope to examine the structure of the disks and outflow regions of massive YSOs in star-forming regions within a few kpc of the Sun. Here we report on 2 micron polarimetry of NGC 6334 V and S255 IRS1. NGC 6334 V consists of a double-lobed bright reflection nebula seen against a dark region, probably an optically thick molecular cloud. Our polarization measurements show that the illuminating star lies ~ 2'' south of the line connecting the two lobes; we do not detect this star at 2 micron, but there are a small radio source and a mid-infrared source at this location. S255 IRS1 consists of two YSOs (NIRS1 and NIRS3) with overlapping scattered light lobes and luminosities corresponding to early B stars. Included in IRS1 is a cluster of stars from whose polarization we determine the local magnetic field direction. Neither YSO has its scattered light lobes aligned with this magnetic field. The line connecting the scattered light lobes of NIRS1 is twisted symmetrically around the star; the best explanation is that the star is part of a close binary and the outflow axis of NIRS1 is precessing as a result of non-coplanar disk and orbit. The star NIRS3 is also offset from the line connecting its two scattered light lobes. We suggest that all three YSOs show evidence of episodic ejection of material as they accrete from dense, optically thick envelopes.

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arXiv2007

Spitzer IRS Observations of the Galactic Center: Shocked Gas in the Radio Arc Bubble

We present Spitzer IRS spectra (R ~600, 10 - 38 micron) of 38 positions in the Galactic Center (GC), all at the same Galactic longitude and spanning plus/minus 0.3 degrees in latitude. Our positions include the Arches Cluster, the Arched Filaments, regions near the Quintuplet Cluster, the ``Bubble'' lying along the same line-of-sight as the molecular cloud G0.11-0.11, and the diffuse interstellar gas along the line-of-sight at higher Galactic latitudes. From measurements of the [O IV], [Ne II], [Ne III], [Si II], [S III], [S IV], [Fe II], [Fe III], and H_2 S(0), S(1), and S(2) lines we determine the gas excitation and ionic abundance ratios. The Ne/H and S/H abundance ratios are ~ 1.6 times that of the Orion Nebula. The main source of excitation is photoionization, with the Arches Cluster ionizing the Arched Filaments and the Quintuplet Cluster ionizing the gas nearby and at lower Galactic latitudes including the far side of the Bubble. In addition, strong shocks ionize gas to O^{+3} and destroy dust grains, releasing iron into the gas phase (Fe/H ~ 1.3 times 10^{-6} in the Arched Filaments and Fe/H ~ 8.8 times 10^{-6} in the Bubble). The shock effects are particularly noticeable in the center of the Bubble, but O$^{+3}$ is present in all positions. We suggest that the shocks are due to the winds from the Quintuplet Cluster Wolf-Rayet stars. On the other hand, the H_2 line ratios can be explained with multi-component models of warm molecular gas in photodissociation regions without the need for H_2 production in shocks.

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arXiv2003

HST/NICMOS Observations of the Embedded Cluster in NGC 2024: Constraints on the IMF and Binary Fraction

We present an analysis of NICMOS observations of the embedded cluster associated with NGC 2024. An analysis of the cluster color-magnitude diagram (CMD) using the models of D'Antona & Mazzitelli (1997) and Baraffe et al. (1998) indicates that the ratio of intermediate mass (1.0 to 10.0 M_sun) to low mass (0.1 to 1.0 M_sun) stars is consistent with the stellar initial mass function (IMF) for the field. In addition to the CMD analysis, we present results on the multiplicity of stars in the region. Three companions (in a sample of 95 potential primaries) were found, with angular separations between 0".4 and 1".0, translating to a projected linear separation of 184 AU to 460 AU for an estimated distance of 460 pc. The completeness of binary detections is assessed using recovery fractions calculated by a series of tests using artificially generated companions to potential primaries in the data frames. We find that the binary fraction in NGC 2024 is consistent with that of Duquennoy & Mayor (1991) for solar neighborhood stars over the range of separations and companion masses appropriate for our survey.

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arXiv2000

The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters

We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R=800) and existing spectral types and photometry and the models of D'Antona & Mazzitelli, we find that the distributions of ages of comparable samples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of \~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass IMFs in these sites of clustered star formation are similar over a wide range of stellar densities and other environmental conditions. With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 Msun, and then rolls over into a power law that continues from about 1 Msun to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 Msun and probably, after a modest completeness correction, even from 0.01-0.02 Msun. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against various properties of the observed IMF.

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