arXiv subjects

Robert D. Blum

Publications and source records attributed to Robert D. Blum.

30 records · Page 2Linked to original sources

arXiv2011

A Population of Accreted SMC Stars in the LMC

We present an analysis of the stellar kinematics of the Large Magellanic Cloud based on ~5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich AGB stars, and other giants. After correcting the line-of-sight velocities for the LMC's space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published HI data. The amplitude of the rotation curve is v_0=87+/-5 km s^-1 beyond a radius R_0=2.4+/-0.1 kpc, and has a position angle of the kinematic line of nodes of theta=142 degrees +/-5 degrees. By examining the outliers from our fits, we identify a population of 376 stars, or >~5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of the LMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with the LMC disk, or rotating in the same sense as the LMC disk, but in a plane that is inclined by 54 degrees +/- 2 degrees to the LMC. Their kinematics clearly link them to two known HI arms, which have previously been interpreted as being pulled out from the LMC. We measure metallicities from the Ca triplet lines of ~1000 LMC field stars and 30 stars in the kinematically distinct population. For the LMC field, we find a median [Fe/H]=-0.56 +/- 0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 +/- 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the SMC. This interpretation has the consequence that the HI arms kinematically associated with the stars are likely falling into the LMC, instead of being pulled out.

astro-ph.GA↗

arXiv2010

The NGC 404 Nucleus: Star Cluster and Possible Intermediate Mass Black Hole

We examine the nuclear morphology, kinematics, and stellar populations in nearby S0 galaxy NGC 404 using a combination of adaptive optics assisted near-IR integral-field spectroscopy, optical spectroscopy, and HST imaging. These observations enable study of the NGC 404 nucleus at a level of detail possible only in the nearest galaxies. The surface brightness profile suggests the presence of three components, a bulge, a nuclear star cluster, and a central light excess within the cluster at radii <3 pc. These components have distinct kinematics with modest rotation seen in the nuclear star cluster and counter-rotation seen in the central excess. Molecular hydrogen emission traces a disk with rotation nearly orthogonal to that of the stars. The stellar populations of the three components are also distinct, with half of the mass of the nuclear star cluster having ages of ~1 Gyr (perhaps resulting from a galaxy merger), while the bulge is dominated by much older stars. Dynamical modeling of the stellar kinematics gives a total nuclear star cluster mass of 1.1x10^7 Msol. Dynamical detection of a possible intermediate mass black hole is hindered by uncertainties in the central stellar mass profile. Assuming a constant mass-to-light ratio, the stellar dynamical modeling suggests a black hole mass of <1x10^5 Msol, while the molecular hydrogen gas kinematics are best fit by a black hole with mass of 4.5x10^5 Msol. Unresolved and possibly variable dust emission in the near-infrared and AGN-like molecular hydrogen emission line ratios do suggest the presence of an accreting black hole in this nearby LINER galaxy.

astro-ph.CO↗

arXiv2009

High Angular Resolution Imaging Spectroscopy of the Galactic Ultra-Compact HII Region K3-50A

Gemini North adaptive optics imaging spectroscopy is presented for the Galactic ultra-compact HII (UCHII) region K3-50A. Data were obtained in the K-band using the Near-infrared Integral Field Spectrograph (NIFS) behind the facility adaptive optics module ALTAIR in natural guide star mode. The NIFS data cube reveals a complex spatial morphology across the 0.1 pc scale of the 3'' UCHII region. Comparison of the nebular emission to Cloudy ionization models shows that the central source must have an effective temperature between about 37000 K and 45000 K with preferred values near 40000 K. Evidence is presented for sharp density variations in the nebula which are interpreted as a clearing of material nearest the central source. High excitation lines of FeIII and SeIV show that the ionization of the nebula clearly changes with distance from the central source. A double lobed kinematic signature (+/- 25 kms) is evident in the Br gamma line map which may be related to the larger scale ionized flow detailed in earlier investigations. This signature and the large scale flow are not co-alighned, but they may still be related. Though clearly resolved from the nebula, the central source itself remains buried, and the NIFS spectrum shows no evidence of photospheric lines.

astro-ph.SR↗

arXiv2009

The Mass-Loss Return From Evolved Stars to the LMC: Empirical Relations for Excess Emission at 8 and 24 μm

We present empirical relations describing excess emission from evolved stars in the Large Magellanic Cloud (LMC) using data from the SAGE (Surveying the Agents of a Galaxy's Evolution) survey which includes the IRAC 3.6, 4.5, 5.8 and 8.0 μm and MIPS 24, 70 and 160 μm bands. We combine the SAGE data with the Two Micron All Sky Survey (2MASS; J, H and Ks) and the optical Magellanic Cloud Photometric Survey (MCPS; U, B, V and I) point source catalogs to create complete spectral energy distributions (SEDs) of the asymptotic giant branch (AGB) star candidates in the LMC. AGB star outflows are among the main producers of dust in a galaxy, and this mass loss results in an excess in the fluxes observed in the 8 and 24 \mic m bands. We identify oxygen-rich, carbon-rich and extreme AGB star populations in our sample based on their 2MASS and IRAC colors. We calculate excesses fluxes in the mid-IR bands by comparison of the SEDs with model photospheres. We find about 16,000 O-rich, 6300 C-rich and 1000 extreme sources with reliable 8 μm excesses, and about 4500 O-rich, 5300 C-rich and 960 extreme sources with reliable 24 \mic m excesses. The excesses are in the range 0.1 mJy--5 Jy. The 8 and 24 \mic m excesses for all three types of AGB candidates show a general increasing trend with luminosity. The dust color temperature derived from the ratio of the 8 and 24 \mic m excesses decreases with an increase in excess, while the 24 \mic m optical depth increases with excess. The extreme AGB candidates are the major contributors to the mass loss, and we estimate the total AGB mass-loss return to the LMC to be (5.9--13)\times 10^{-3} Msun yr^{-1}.

astro-ph.CO↗

arXiv2008

The Rotating Nuclear Star Cluster in NGC 4244

We present observations of the nuclear star cluster in the nearby edge-on spiral galaxy NGC 4244 using the Gemini Near-Infrared Integral Field Spectrograph (NIFS) with laser guide star adaptive optics. From a previous study of edge-on galaxies, this nuclear star cluster was found to be one of a sample of clusters that appear flattened along the plane of their host galaxies disks. Such clusters show evidence for multiple morphological components, with younger/bluer disk components and older/redder spheroidal components. Our new observations of NGC 4244 show clear rotation of 30 km/sec within the central 10 pc (0.5") of the cluster. The central velocity dispersion is found to be 28+/-2 km/sec. The multiple stellar populations inferred from the optical colors and spectra are seen as variations in the CO line strength in the NIFS spectra. The rotation is clearly detected even in the older, more spheroidal stellar component. We discuss evidence for similar structures and kinematics in the nuclear star clusters of other galaxies including M33 and the Milky Way. Our observations support two possible formation mechanisms: (1) episodic accretion of gas from the disk directly onto the nuclear star cluster, or (2) episodic accretion of young star clusters formed in the central part of the galaxy due to dynamical friction.

astro-ph↗

arXiv2008

The Ionizing Stars of the Galactic Ultra-Compact HII Region G45.45+0.06

Using the NIFS near-infrared integral-field spectrograph behind the facility adaptive optics module, ALTAIR, on Gemini North, we have identified several massive O-type stars that are responsible for the ionization of the Galactic Ultra-Compact HII region G45.45+0.06. The sources ``m'' and ``n'' from the imaging study of Feldt et a. 1998 are classified as hot, massive O-type stars based on their K-band spectra. Other bright point sources show red and/or nebular spectra and one appears to have cool star features that we suggest are due to a young, low-mass pre-main sequence component. Still two other embedded sources (``k'' and ``o'' from Feldt et al.) exhibit CO bandhead emission that may arise in circumstellar disks which are possibly still accreting. Finally, nebular lines previously identified only in higher excitation planetary nebulae and associated with KrIII and SeIV ions are detected in G45.45+0.06.

astro-ph↗

arXiv2007

Chemical Abundances of Luminous Cool Stars in the Galactic Center from High-Resolution Infrared Spectroscopy

We present chemical abundances in a sample of luminous cool stars located within 30 pc of the Galactic Center. Abundances of carbon, nitrogen, oxygen, calcium, and iron were derived from high-resolution infrared spectra in the H- and K-bands. The abundance results indicate that both [O/Fe] and [Ca/Fe] are enhanced respectively by averages of +0.2 and +0.3 dex, relative to either the Sun or the Milky Way disk at near solar Fe abundances. The Galactic Center stars show a nearly uniform and nearly solar iron abundance. The mean value of A(Fe) = 7.59 +- 0.06 agrees well with previous work. The total range in Fe abundance among Galactic Center stars, 0.16 dex, is significantly narrower than the iron abundance distributions found in the literature for the older bulge population. Our snapshot of the current-day Fe abundance within 30 pc of the Galactic Center samples stars with an age less than 1 Gyr; a larger sample in time (or space) may find a wider spread in abundances.

astro-ph↗

arXiv2006

The Star Formation Histories of the Bulge and Disk of M31 from Gemini North+NIRI/Altair and HST/NICMOS

We discuss H and K observations of three fields in the bulge and disk of M31 obtained with the Altair adaptive optics system and NIRI instrument on Gemini North. These are the highest resolution and deepest near-infrared observations obtained to date of the inner regions of M31, and demonstrate the promise of ground-based adaptive optics for studying the crowded regions of nearby galaxies. We have combined our observations with previously published HST/NICMOS observations of nine M31 fields and have derived the coarse star formation histories of M31's bulge and inner disk. From fits to the M_K luminosity functions, we find the stellar population mix to be dominated by old, nearly solar-metallicity stars. The old populations, which we define as having age >~6 Gyr, indeed dominate the star formation histories at all radii independent of the relative contributions of bulge and disk stars. Although all of our fields contain some bulge contribution, our results suggest that there is no age difference between the bulge and disk to the limit of our precision.

astro-ph↗

arXiv2003

High-Resolution Infrared Spectroscopy of the Brown Dwarf Epsilon Indi Ba

We report on the analysis of high-resolution infrared spectra of the newly discovered brown dwarf Epsilon Indi Ba. This is the closest known brown dwarf to the solar system, with a distance of 3.626 pc. Spectra covering the ranges of 2.308-2.317 microns and 1.553-1.559 microns were observed at a spectral resolution of R=50,000 with the Phoenix spectrometer on the Gemini South telescope. The physical paramters of effective temperature and surface gravity are derived by comparison to model spectra calculated from atmospheres computed using unified cloudy models. An accurate projected rotational velocity is also derived.

astro-ph↗

arXiv2003

Stellar Crowding and the Science Case for Extremely Large Telescopes

We present a study of the effect of crowding on stellar photometry. We develop an analytical model through which we are able to predict the error in magnitude and color for a given star for any combination of telescope resolution, stellar luminosity function, background surface brightness, and distance. We test our predictions with Monte Carlo simulations of the LMC globular cluster NGC 1835, for resolutions corresponding to a seeing-limited telescope, the $HST$, and an AO-corrected 30-m (near diffraction limited) telescope. Our analytically predicted magnitude errors agree with the simulation results to within $\sim$20%. The analytical model also predicts that errors in color are strongly affected by the correlation of crowding--induced photometric errors between bands as is seen in the simulations. Using additional Monte Carlo simulations and our analytical crowding model, we investigate the photometric accuracy which 30-m and 100-m Extremely Large Telescopes (ELTs) will be able to achieve at distances extending to the Virgo cluster. We argue that for stellar populations work, ELTs quickly become crowding-limited, suggesting that low--Strehl AO systems may be sufficient for this type of science.

astro-ph↗

arXiv2001

Massive Star Birth in the Inner Galaxy: Obscured Massive Star Clusters

The current census of, and stellar population in, massive Galactic star clusters is reviewed. In particular, we concentrate on a recent survey of obscured Galactic Giant H II (GHII) regions and the associated stellar clusters embedded in them. The regions have been selected as the most luminous radio continuum sources, and as such the stellar clusters appear to be among the youngest massive clusters in the Galaxy. The emergent stellar populations are further studied through near infrared spectroscopy of the brighter members. We also discuss the massive stellar clusters within 50 pc of the Galactic center (GC), comparing their known properties to those found in the GHII region survey. It is suggested that the somewhat younger clusters associated with the GHII regions are more suited to measuring the initial mass function in massive star clusters. Narrow band images in the central pc of the GC are presented which identify the young stellar sequence associated with the evolved He I emission line stars.

astro-ph↗

arXiv2001

Near Infrared Observations of the Giant HII Region W49A: A Starbirth Cluster

W49A is one of the most luminous giant H II (GH II) regions in our Galaxy. This star forming complex contains numerous compact and ultra-compact (UC) H II regions, extending over an area of 15 pc. It emits about 10^{51} Lyman continuum photons per second, equivalent to the presence of about 100 O stars, but it is completely obscured in optical wavelengths by intervening interstellar dust. The center holds a ``cluster'' of about 30 O stars, each within an individual UCHII region emitting free-free emission at cm wavelengths. Our deep K-band (2.2 um) image toward the W49A cluster reveals just two of the individual exciting stars, each associated with a point--like radio source, but the rest are invisible. These O stars are so recently born as to not yet have emerged from their natal dust cocoons, in contrast to other Galactic clusters embedded in GH II regions in which many of the individual massive stars are already revealed. Plausibility arguments are made which suggest that a stellar disc might be common during the entire UCH II phase of massive star birth, as it persists after accretion ceases in some stars. Nebular emission (e.g., from Br gamma) is visible around the periphery of the central region of W49A, along with candidate exciting stars. Star formation there may have preceeded that in the center, or its lower density environment may have speeded up the dispersal of the natal dust cocoons. The W49A cluster can serve as a template for the more luminous buried star clusters now being found in normal galaxies and starbursts.

astro-ph↗