19 August 2012

NSF AST Portfolio Review

Recently, amid US government budget shortcomings, the National Science Foundation (NSF) convened a panel to review the research priorities set forth by the 2010 Decadal Review, otherwise known as Astro2010. The NSF AST Portfolio Review reassessed the research priorities and objectives for the US ground-based observatories which carries potentially large implications for the astronomical community if or when the recommendations are upheld. Included in this, was the recommendation to "divest" in resources such as the WIYN and 2.1 meter telescopes atop Kitt Peak. However, a number of facilities were recommended for continued support, most notably ALMA and the VLA. For those interested, the full document is freely available online.

Given that there has been some time for people to read the document, or various blog posts summarizing it, what are you thoughts? Do you feel the budget allocation recommendations were adequate, or were there certain projects recommended for divestment that you were hoping to see remain open? Finally, how does the recommended budget allocations affect your current research or your future research plans, if at all?

14 February 2012

Naked Stellar Core

Title: Discovery of a stripped red giant core in a bright eclipsing binary star
Authors P.F.L. Maxted, D.R. Anderson, M.R. Burleigh, et al.

Before delving into the topic, it should first be pointed out that the discovery paper was first published in the Monthly Notices in September of 2011. The article can be found here.

Figure 1. Phase folded light curve for 1SWASP J024743.37-251549.2.
The title of the article is very effective at describing the system presented in this paper. Maxted et al. announced the discovery of an eclipsing binary system in which the primary star completely occults the secondary star. However, the secondary star is found to have a higher effective temperature than the primary star. Photometric analysis suggests the primary is an A star and that it contributes approximately 90% of the total flux of the system.

Figure 1 neatly elucidates this information. The deeper eclipse (at phase 0) indicates when the cooler star is passing between our line-of-sight and the hotter star. However, the fact that the eclipse profile is flat at the bottom implies the hotter star is being completely occulted, yet the total flux received from the system is hardly affected.

To further complicate things, a rough age estimate can be obtained from the system's kinematics. Space motions indicate the system is a part of the galactic thick disk, meaning the system has undergone significant disk heating and has acquired a larger vertical component to it's motion than would be expected from a young system forming in the galactic thin disk. The characteristic age for the galactic thick disk is > ~7 Gyr meaning the system has an age well older than the lifetime of an A star!

So what is going on here? Numerical modeling has shown that the system is consistent with a red giant that has had its outer layers stripped off, leaving behind a He core with a H envelope. There is some degree of H burning taking place in a shell around the inert He core. How was that mass stripped off and where did it end up?

Since the secondary was plausibly a red giant before it had mass stripped away and since the primary is, as far as we can tell, a normal main sequence star, the secondary star must be more massive to be in a more advanced evolutionary stage. As the more massive star puffed up when it started to become a red giant, it overflowed it's Roche lobe and began funneling material to its lower mass companion. Eventually the mass transfer halted and left the system in the state we find it today.

Stellar evolution dictates that a star will begin it's ascent up the red giant branch once it has exhausted most of the hydrogen in its core. However, the core is not yet hot enough to ignite helium burning (hence the core contracts and the exterior inflates to conserve flux), leaving the star with an inert He core. It will live out the rest of it's life as a He white dwarf once the H shell burning halts (pre-He-WD). As for the star that received the additional mass, it is now living the life of a higher mass star (hence the A spectral type). These particular stars are known as blue stragglers.

The system is rare and exciting, but the WASP team has indicated they have more examples of blue straggler/pre-He-WD systems. Stay tuned and keep an eye out for more of these remarkable systems!

17 January 2012

Circumgalactic Media and Their Hosts

Title: The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals
Authors: J. Tumlinson et al.

Galaxies grow and evolve by accreting gas from the intergalactic medium (IGM), forming stars with this material, and ejecting the often-enriched remnants through galactic-scale outflows.  At the convergence of these processes lies the circumgalactic medium (CGM), gas surrounding galaxies out to 100 to 300 kpc.  This paper investigates the relationship between properties of host galaxies and their CGM using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope with absorption-line spectroscopy.

The method of measuring absorption lines through CGM is as follows.  The study focuses on 42 sample galaxies that are close to distant QSOs on the plane of the sky.  As light from the QSOs passes through the CGM of the galaxies, absorption lines can be measured, specifically the ultraviolet O VI doublet 1032 and 1038.  The data is (are?) used to measure O VI column densities, line profiles, and radial velocities of the CGM with respect to the host galaxies.  Furthermore, the Keck Observatory Low-Resolution Imaging Spectrograph (LRIS) and the Las Campanas Observatory Magellan Echellette (MagE) spectrograph were used to measure redshift, star formation rate, and metallicity for each galaxy.

Not surprisingly, the study found that each CGM was close in radial velocity to its host galaxy, suggesting a close physical and gravitational relationship.  Furthermore, there is a correlation between O VI column density and specific star formation rate (sSFR): active, star-forming galaxies have much higher column densities than passive galaxies.  This reflects the bimodality of galaxies and suggests that the CGM either directly affects or is affected directly by the galaxy's star formation.

The CGM also contains a substantial fraction of the metals in the galaxy, and the ratio of CGM metals to ISM metals increases with decreasing galaxy mass.  These metals were most likely created in the galaxies and then transported into the CGM by outflows.  Taking into account the amount of oxygen returned to the ISM during star formation and the typical star formation rate, the authors estimate that the oxygen in the CGM could have been deposited there over several billion years of star formation and outflow.  Furthermore, the observed O VI outflows do not exceed the galaxies' escape velocities, suggesting that this enrichment could eventually fall back onto the galaxy to fuel further star formation.