Thursday August 29th, 2013

Recently in our circle we have been repeatedly discussing that the centers of galaxies are typically dominated by older stellar populations, resulting in a very ‘red’ energy distribution. However, Dewitt et. al. uses near-infrared spectroscopy to identify three x-ray sources near the galactic center as high mass stars/binaries.

The next paper by Giardi et. al. suggests that the contributions of Thermally Pulsating Asymptotic Giant Branch (TP-AGB) in cluster models may be overestimated by a factor of two. Could this possibly have implications for dust production models relying on such stars? Either way, this paper is somewhat amusing for its rather colorful language.

Finally, Howard et. al. presents some [OI] and [CII] line data from Herschel on protoplanetary disks.

Three New Galactic Center X-ray Sources Identified with Near-Infrared Spectroscopy

Curtis DeWitt, Reba M. Bandyopadhyay, Stephen S. Eikenberry, Kris Sellgren, Robert Blum, Knut Olsen, Franz E. Bauer, Ata Sarajedini
(Submitted on 28 Aug 2013)

We have conducted a near-infrared spectroscopic survey of 47 candidate counterparts to X-ray sources discovered by the Chandra X-ray Observatory near the Galactic Center (GC). Though a significant number of these astrometric matches are likely to be spurious, we sought out spectral characteristics of active stars and interacting binaries, such as hot, massive spectral types or emission lines in order to corroborate the X-ray activity and certify the authenticity of the match. We present three new spectroscopic identifications, including a Be high mass X-ray binary (HMXB) or a gamma Cassiopeiae (Cas) system, a symbiotic X-ray binary and an O-type star of unknown luminosity class. The Be HMXB/gamma Cas system and the symbiotic X-ray binary are the first of their classes to be spectroscopically identified in the GC region.

Comments: 33 pages, 12 figures, AJ accepted
Subjects: Galaxy Astrophysics (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)
Cite as: arXiv:1308.6252 [astro-ph.GA]
(or arXiv:1308.6252v1 [astro-ph.GA] for this version)

The insidious boosting of TP-AGB stars in intermediate-age Magellanic Cloud clusters

Leo Girardi, Paola Marigo, Alessandro Bressan, Philip Rosenfield
(Submitted on 28 Aug 2013)

(Abridged) In the recent controversy about the role of TP-AGB stars in evolutionary population synthesis (EPS) models of galaxies, one particular aspect is puzzling: TP-AGB models aimed at reproducing the lifetimes and integrated fluxes of the TP-AGB phase in Magellanic Cloud (MC) clusters, when incorporated into EPS models, are found to overestimate the TP-AGB contribution in resolved star counts and integrated spectra of galaxies. In this paper, we call attention to a particular evolutionary aspect that in all probability is the main cause of this conundrum. As soon as stellar populations intercept the ages at which RGB stars first appear, a sudden change in the lifetime of the core He-burning phase causes a temporary boost in the production rate of subsequent evolutionary phases, including the TP-AGB. For a timespan of about 0.1 Gyr, triple TP-AGB branches develop at slightly different initial masses, causing their frequency and contribution to the integrated luminosity of the stellar population to increase by a factor of 2. The boost occurs just in the proximity of the expected peak in the TP-AGB lifetimes, and for ages of 1.6 Gyr. Coincidently, this relatively narrow age interval happens to contain the few very massive MC clusters that host most of the TP-AGB stars used to constrain stellar evolution and EPS models. This concomitance makes the AGB-boosting particularly insidious in the context of present EPS models. The effect brings about three main consequences. (1) Present estimates of the TP-AGB contribution to the integrated light of galaxies derived from MC clusters, are biased towards too large values. (2) The relative TP-AGB contribution of single-burst populations falling in this critical age range cannot be accurately derived by the fuel consumption theorem. (3) A careful revision of AGB star populations in intermediate-age MC clusters is urgently demanded.

Comments: ApJ accepted, 12 pages
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:1308.6088 [astro-ph.SR]
(or arXiv:1308.6088v1 [astro-ph.SR] for this version)

Herschel/PACS Survey of protoplanetary disks in Taurus/Auriga — Observations of [OI] and [CII], and far infrared continuum

Christian D. Howard (1,2), Göran Sandell (1), William D.Vacca (1), Gaspard Duchêne (3,4), Geoffrey Mathews (5,6), Jean-Charles Augereau (4), David Barrado (7,8),William R. F.Dent (9), Carlos Eiroa (10), Carol Grady (11,12,13), Inga Kamp (14), Gwendolyn Meeus (10), Francois Ménard (4,15), Christophe Pinte (4), Linda Podio (4),Pablo Riviere-Marichalar (7), Aki Roberge (11), Wing-Fai Thi (4), Silvia Vicente (14), Jonathan P. Williams (6) ((1) SOFIA-USRA, NASA Ames Research Center (2) Google (3) UC Berkeley (4) UJF-Grenoble (5) Leiden Observatory (6) IfA, University of Hawaii (7) Centro de Astrobiologia, ESAC (8) Calar Alto Observatory (9) ALMA SCO (10) Dep de Fisica Theoretica, UAM (11) Exoplanets and Stellar Astrophys. Lab, NASA Goddard (12) Eureca Scientific (13) Kapteyn Astronomical Institute (14) UMI-FCA, France and Univ. de Chile)
(Submitted on 28 Aug 2013)

The Herschel Space Observatory was used to observe ~ 120 pre-main-sequence stars in Taurus as part of the GASPS Open Time Key project. PACS was used to measure the continuum as well as several gas tracers such as [OI] 63 \mu m, [OI] 145 \mu m, [CII] 158 \mu m, OH, H2O and CO. The strongest line seen is [OI] at 63 \mu m. We find a clear correlation between the strength of the [OI] 63 \mu m line and the 63 \mu m continuum for disk sources. In outflow sources, the line emission can be up to 20 times stronger than in disk sources, suggesting that the line emission is dominated by the outflow. The tight correlation seen for disk sources suggests that the emission arises from the inner disk ($<$ 50 AU) and lower surface layers of the disk where the gas and dust are coupled. The [OI] 63 \mu m is fainter in transitional stars than in normal Class II disks. Simple SED models indicate that the dust responsible for the continuum emission is colder in these disks, leading to weaker line emission. [CII] 158 \mu m emission is only detected in strong outflow sources. The observed line ratios of [OI] 63 \mu m to [OI] 145 \mu m are in the regime where we are insensitive to the gas-to-dust ratio, neither can we discriminate between shock or PDR emission. We detect no Class III object in [OI] 63 \mu m and only three in continuum, at least one of which is a candidate debris disk.

Comments: 84 pages, 14 figures, 5 tables. Accepted by ApJ
Subjects: Galaxy Astrophysics (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:1308.6019 [astro-ph.GA]
(or arXiv:1308.6019v1 [astro-ph.GA] for this version)

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