arXiv subjects

Uma P. Vijh

Publications and source records attributed to Uma P. Vijh.

15 recordsLinked to original sources

arXiv2011

The nature of the Na I D-lines in the Red Rectangle

In this paper we examine the profiles of the complex Na I D-lines in the Red Rectangle. The spectra were acquired with the ARCES échelle spectrograph R = 38,000 on the 3.5-m telescope at the Apache Point Observatory. Additional spectra taken with STIS were acquired from the Hubble Legacy Archive (HLA) and were used to independently confirm the spatial origin of the spectral features. The profile of a single D-line consists of double-peaked emission, red-shifted absorption and blue-shifted absorption. We find that the double-peaked emission originates from the bipolar outflow, the red-shifted absorption feature is due to the photospheric line, and the blue-shifted absorption arises from the bipolar outflow as seen against the photosphere of the luminous post-AGB component in HD 44179. In order to better understand the Na I D-line profile, we examined the periodically variable asymmetric photospheric absorption lines. The asymmetric lines are interpreted as a signature of slow self-accretion following enhanced mass-loss around periastron. An empirical model was constructed to remove the photospheric component from the Na I D-line profile in order to study the nebular emission and absorption of sodium along the line-of-sight to the primary. This paper also discusses the different origins of the single-peaked emission, the double-peaked emission and the blue-shifted and red-shifted absorption components.

astro-ph.SR↗

arXiv2011

Surveying the Agents of Galaxy Evolution in the Tidally-Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). I. Overview

The Small Magellanic Cloud (SMC) provides a unique laboratory for the study of the lifecycle of dust given its low metallicity (~1/5 solar) and relative proximity (~60 kpc). This motivated the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the Tidally-Stripped, Low Metallicity Small Magellanic Cloud) Spitzer Legacy program with the specific goals of studying the amount and type of dust in the present interstellar medium, the sources of dust in the winds of evolved stars, and how much dust is consumed in star formation. This program mapped the full SMC (30 sq. deg.) including the Body, Wing, and Tail in 7 bands from 3.6 to 160 micron using the IRAC and MIPS instruments on the Spitzer Space Telescope. The data were reduced, mosaicked, and the point sources measured using customized routines specific for large surveys. We have made the resulting mosaics and point source catalogs available to the community. The infrared colors of the SMC are compared to those of other nearby galaxies and the 8 micron/24 micron ratio is somewhat lower and the 70 micron/160 micron ratio is somewhat higher than the average. The global infrared spectral energy distribution shows that the SMC has ~3X lower aromatic emission/PAH (polycyclic aromatic hydrocarbon) abundances compared to most nearby galaxies. Infrared color-magnitude diagrams are given illustrating the distribution of different asymptotic giant branch stars and the locations of young stellar objects. Finally, the average spectral energy distribution (SED) of HII/star formation regions is compared to the equivalent Large Magellanic Cloud average HII/star formation region SED. These preliminary results are expanded in detail in companion papers.

astro-ph.CO↗

arXiv2010

The SAGE-Spec Spitzer Legacy program: The life-cycle of dust and gas in the Large Magellanic Cloud. Point source classification I

We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy program on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership, and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two HII regions and 12 other objects, seven of which remain unclassified.

astro-ph.GA↗

arXiv2010

Spitzer Analysis of HII Region Complexes in the Magellanic Clouds: Determining a Suitable Monochromatic Obscured Star Formation Indicator

HII regions are the birth places of stars, and as such they provide the best measure of current star formation rates (SFRs) in galaxies. The close proximity of the Magellanic Clouds allows us to probe the nature of these star forming regions at small spatial scales. We aim to determine the monochromatic IR band that most accurately traces the bolometric IR flux (TIR), which can then be used to estimate an obscured SFR. We present the spatial analysis, via aperture/annulus photometry, of 16 LMC and 16 SMC HII region complexes using the Spitzer IRAC and MIPS bands. UV rocket data and SHASSA H-alpha data are also included. We find that nearly all of the LMC and SMC HII region SEDs peak around 70um, from ~10 to ~400 pc from the central sources. As a result, the sizes of HII regions as probed by 70um is approximately equal to the sizes as probed by TIR (about 70 pc in radius); the radial profile of the 70um flux, normalized by TIR, is constant at all radii (70um ~ 0.45 TIR); the 1-sigma standard deviation of the 70um fluxes, normalized by TIR, is a lower fraction of the mean (0.05 to 0.12 out to ~220 pc) than the normalized 8, 24, and 160um normalized fluxes (0.12 to 0.52); and these results are invariant between the LMC and SMC. From these results, we argue that 70um is the most suitable IR band to use as a monochromatic obscured star formation indicator because it most accurately reproduces the TIR of HII regions in the LMC and SMC and over large spatial scales. We also explore the general trends of the 8, 24, 70, and 160um bands in the LMC and SMC HII region SEDs, radial surface brightness profiles, sizes, and normalized (by TIR) radial flux profiles. We derive an obscured SFR equation that is modified from the literature to use 70um luminosity, SFR(Mo/yr) = 9.7(0.7)x10^{-44} L(70)(ergs/s), which is applicable from 10 to 300 pc distance from the center of an HII region.

astro-ph.CO↗

arXiv2010

The SAGE-Spec Spitzer Legacy program: The life-cycle of dust and gas in the Large Magellanic Cloud

The SAGE-Spec Spitzer Legacy program is a spectroscopic follow-up to the SAGE-LMC photometric survey of the Large Magellanic Cloud carried out with the Spitzer Space Telescope. We present an overview of SAGE-Spec and some of its first results. The SAGE-Spec program aims to study the life cycle of gas and dust in the Large Magellanic Cloud, and to provide information essential to the classification of the point sources observed in the earlier SAGE-LMC photometric survey. We acquired 224.6 hours of observations using the InfraRed Spectrograph and the SED mode of the Multiband Imaging Photometer for Spitzer. The SAGE-Spec data, along with archival Spitzer spectroscopy of objects in the Large Magellanic Cloud, are reduced and delivered to the community. We discuss the observing strategy, the specific data reduction pipelines applied and the dissemination of data products to the scientific community. Initial science results include the first detection of an extragalactic "21 um" feature towards an evolved star and elucidation of the nature of disks around RV Tauri stars in the Large Magellanic Cloud. Towards some young stars, ice features are observed in absorption. We also serendipitously observed a background quasar, at a redshift of z~0.14, which appears to be host-less.

astro-ph.GA↗

arXiv2009

Spatial variations of dust abundances across the Large Magellanic Cloud

Using the data obtained with the Spitzer Space telescope as part of the Surveying the Agents of a Galaxy's Evolution (SAGE) legacy survey, we have studied the variations of the dust composition and abundance across the Large Magellanic Cloud (LMC). Such variations are expected, as the explosive events which have lead to the formation of the many HI shells observed should have affected the dust properties. Using a model and comparing with a reference spectral energy distribution from our Galaxy, we deduce the relative abundance variations of small dust grains across the LMC. We examined the infrared color ratios as well as the relative abundances of very small grains (VSGs) and polycyclic aromatic hydrocarbons (PAHs) relative to the big grain (BG) abundance. Results show that each dust component could have different origins or evolution in the interstellar medium (ISM). The VSG abundance traces the star formation activity and could result from shattering of larger grains, whereas the PAH abundance increases around molecular clouds as well as in the stellar bar, where they could have been injected into the ISM during mass loss from old stars.

astro-ph.GA↗

arXiv2008

The Red Rectangle: Its Shaping Mechanism and its Source of Ultraviolet Photons

The proto-planetary Red Rectangle nebula is powered by HD 44179, a spectroscopic binary (P = 318 d), in which a luminous post-AGB component is the primary source of both luminosity and current mass loss. Here, we present the results of a seven-year, eight-orbit spectroscopic monitoring program of HD 44179, designed to uncover new information about the source of the Lyman/far-ultraviolet continuum in the system as well as the driving mechanism for the bipolar outflow producing the current nebula. Our observations of the H-alpha line profile around the orbital phase of superior conjunction reveal the secondary component to be the origin of the fast (max. v~560$ km s$^{-1}$) bipolar outflow in the Red Rectangle. The variation of total H-alpha flux from the central H II region with orbital phase also identifies the secondary or its surroundings as the source of the far-ultraviolet ionizing radiation in the system. The estimated mass of the secondary (~0.94 M$\sun$) and the speed of the outflow suggest that this component is a main sequence star and not a white dwarf, as previously suggested. We identify the source of the Lyman/far-ultraviolet continuum in the system as the hot, inner region (T$_{max} \ge 17,000$ K) of an accretion disk surrounding the secondary, fed by Roche lobe overflow from the post-AGB primary at a rate of about $2 - 5\times10^{-5}$ M$\sun$ yr$^{-1}$. The total luminosity of the accretion disk around the secondary is estimated to be at least 300 L$\sun$, about 5% of the luminosity of the entire system. (abridged)

astro-ph↗

arXiv2008

Variable Evolved Stars and YSOs Discovered in the Large Magellanic Cloud using the SAGE Survey

We present initial results and source lists of variable sources in the Large Magellanic Cloud (LMC) for which we detect thermal infrared variability from the SAGE (Surveying the Agents of a Galaxy's Evolution) survey, which had 2 epochs of photometry separated by three months. The SAGE survey mapped a 7 degree by 7 degree region of the LMC using the IRAC and the MIPS instruments on board Spitzer. Variable sources are identified using a combination of the IRAC 3.6, 4.5, 5.8, 8.0 \micron bands and the MIPS 24 \micron bands. An error-weighted flux difference between the two epochs is used to assess the variability. Of the ~ 3 million sources detected at both epochs we find ~ 2,000 variable sources for which we provide electronic catalogs. Most of the variable sources can be classified as asymptotic giant branch (AGB) stars. A large fraction (> 66%) of the extreme AGB stars are variable and only smaller fractions of carbon-rich (6.1%) and oxygen-rich (2.0%) stars are detected as variable. We also detect a population of variable young stellar object candidates.

astro-ph↗

arXiv2006

Optical Emission Band Morphologies of the Red Rectangle

We present narrow-band images of the Red Rectangle (RR) nebula which reveal the distinct morphologies of this intriguing nebula in different optical emission bands. The morphology of the RR nebula in blue luminescence (BL) and extended red emission (ERE) are almost mutually exclusive. We also present the optical detection of the circum-binary disk of the RR in the light of the BL. The total intensities from the two optical band emissions (BL and ERE) when summed over the nebula are of comparable magnitude. Their spatial distributions with respect to the embedded illumination sources lead us to suggest that they may be attributed to different ionization stages of the same family of carriers.

astro-ph↗

arXiv2005

The Excitation of Extended Red Emission: New Constraints on its Carrier From HST Observations of NGC 7023

The carrier of the dust-associated photoluminescence process causing the extended red emission (ERE) in many dusty interstellar environments remains unidentified. Several competing models are more or less able to match the observed broad, unstructured ERE band. We now constrain the character of the ERE carrier further by determining the wavelengths of the radiation that initiates the ERE. Using the imaging capabilities of the Hubble Space Telescope, we have resolved the width of narrow ERE filaments appearing on the surfaces of externally illuminated molecular clouds in the bright reflection nebula NGC 7023 and compared them with the depth of penetration of radiation of known wavelengths into the same cloud surfaces. We identify photons with wavelengths shortward of 118 nm as the source of ERE initiation, not to be confused with ERE excitation, however. There are strong indications from the well-studied ERE in the Red Rectangle nebula and in the high-|b| Galactic cirrus that the photon flux with wavelengths shortward of 118 nm is too small to actually excite the observed ERE, even with 100% quantum efficiency. We conclude, therefore, that ERE excitation results from a two-step process. While none of the previously proposed ERE models can match these new constraints, we note that under interstellar conditions most polycyclic aromatic hydrocarbon (PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV and that the electronic energy level structure of PAH di-cations is consistent with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations deserve further study as potential carriers of the ERE. (abridged)

astro-ph↗

arXiv2005

Blue Luminescence and the Presence of Small PAHs in the ISM

Blue Luminescence (BL) was first discovered in a proto-planetary nebula, the Red Rectangle (RR) surrounding the post-AGB star HD 44179. BL has been attributed to fluorescence by small, 3-4 ringed neutral polycyclic aromatic hydrocarbon (PAH) molecules, and was thought to be unique to the RR environment where such small molecules are actively being produced and shielded from the harsh interstellar radiation by a dense circumstellar disk. In this paper we present the BL spectrum detected in several ordinary reflection nebulae illuminated by stars having temperatures between 10,000 -- 23,000 K. All these nebulae are known to also exhibit the infrared emission features called aromatic emission features (AEFs) attributed to large PAHs. We present the spatial distribution of the BL in these nebulae. In the case of Ced~112, the BL is spatially correlated with mid-IR emission structures attributed to AEFs. These observations provide evidence for grain processing and possibly for in-situ formation of small grains and large molecules from larger aggregates. Most importantly, the detection of BL in these ordinary reflection nebulae suggests that the BL carrier is an ubiquitous component of the ISM and is not restricted to the particular environment of the RR.

astro-ph↗

arXiv2004

Small PAHs in the Red Rectangle

Following our initial discovery of blue luminescence in the spectrum of the Red Rectangle (RR) and its identification as fluorescence by small three- to four-ringed polycyclic aromatic hydrocarbon (PAH) molecules, we report on the spatial correlation between the blue luminescence and the 3.3 micron emission, commonly attributed to small, neutral PAH molecules, and on the newly-derived UV/optical attenuation curve for the central source of the RR, HD 44179. Both results provide strong additional evidence for the presence of small PAH molecules with masses of less than 250 a.m.u. in the RR, which supports the attribution of the blue luminescence to fluorescence by the same molecules. We contrast the excellent spatial correlation of the two former emissions with the distinctly different spatial distribution of the extended red emission (ERE) and of the dust-scattered light within the RR. The UV/optical attenuation curve of the central star is unlike any interstellar extinction curve and is interpreted as resulting from circumstellar opacity alone. Major contributions to this opacity are absorptions in broad bands in the mid-UV, contributing to the electronic excitation of the luminescing PAH molecules, and a sharp ionization discontinuity near 7.5 eV in the far-UV, which places a sharp upper limit on the masses of the PAH molecules that are responsible for this absorption. The strength of the far-UV absorption leads to an abundance of the PAH molecules of 10^{-5} relative to hydrogen in the RR. Such small PAHs are perhaps unique to the environment in the RR, where they are shielded from harsh radiation by the dense circmstellar disk.

astro-ph↗

arXiv2004

Discovery of Blue Luminescence in the Red Rectangle: Possible Fluorescence from Neutral Polycyclic Aromatic Hydrocarbon Molecules?

Here we report our discovery of a band of blue luminescence (BL) in the Red Rectangle (RR) nebula. This enigmatic proto-planetary nebula is also one of the brightest known sources of extended red emission as well as of unidentified infra-red (UIR) band emissions. The spectrum of this newly discovered BL is most likely fluorescence from small neutral polycyclic aromatic hydrocarbon (PAH) molecules. PAH molecules are thought to be widely present in many interstellar and circumstellar environments in our galaxy as well as in other galaxies, and are considered likely carriers of the UIR-band emission. However, no specific PAH molecule has yet been identified in a source outside the solar system, as the set of mid-infra-red emission features attributed to these molecules between the wavelengths of 3.3 micron and 16.4 micron is largely insensitive to molecular sizes. In contrast, near-UV/blue fluorescence of PAHs is more specific as to size, structure, and charge state of a PAH molecule. If the carriers of this near-UV/blue fluorescence are PAHs, they are most likely neutral PAH molecules consisting of 3-4 aromatic rings such as anthracene (C14H10) and pyrene (C16H10). These small PAHs would then be the largest molecules specifically identified in the interstellar medium.

astro-ph↗

arXiv2003

Extended Red Emission: Photoluminescence by Interstellar Nanoparticles

Interstellar dust in nebulae and in the diffuse interstellar medium of galaxies contains a component which responds to illumination by ultraviolet photons with efficient luminescence in the 500 nm to 1000 nm spectral range, known as Extended Red Emission (ERE). We review the techniques of detection and the observational characteristics of the ERE in a wide range of astrophysical environments. We then discuss results of the analysis of ERE observations, leading to a set of specific constraints that any proposal for the ERE carrier must confront. Finally, we review specific models that have been advanced over the past two decades to explain the ERE phenomenon. Despite promising progress on several fronts, no completely satisfactory model for the ERE carrier/process exists at this time.

astro-ph↗

arXiv2003

The Dust in Lyman Break Galaxies

We present our analysis of UV attenuation by internal dust of a large sample (N=906 galaxies) of Lyman Break Galaxies (LBGs). Using spectral energy distributions (SEDs) from the PÉGASE galaxy spectral evolution model we apply dust attenuation corrections to the G-R colors using the Witt & Gordon (2000) models for radiative transfer in dusty galactic environments to arrive at the UV attenuation factors. We show that the dust in the LBGs exhibit SMC-like characteristics rather than MW-like, and that the dust geometry in these systems is most likely to be represented by a clumpy shell configuration. We show that the attenuation factor exhibits a pronounced dependence on the luminosity of the LBG, $a_{1600}\propto (L/L_\sun)^α$, where $0.5\leqα\leq1.5$. The exponent $α$ depends on the initial parameters of the stellar population chosen to model the galaxies and the dust properties. We find that the luminosity weighted average attenuation factor is likely to be in the range from $5.7-18.5$, which is consistent with the upper limits to the star formation rate at $2<z<4$ set by the FIR background. This implies that the current UV/optical surveys do detect the bulk of the star formation during the epoch $2<z<4$, but require substantial correction for internal dust attenuation.

astro-ph↗