Research

Overview

Hot Subdwarfs -- sdB Stars

Gravity Modes in the Long-Period Pulsating Subdwarf B Star

Monte Carlo Simulations of Post-Common Envelope SdB Star Plus White Dwarf Binaries

Modeling System Parameters of KBS 13 -- a Rare Reflection Effect of sdB Binary with an M dwarf Secondary

Metal-Poor Stars

Searching for Alpha-Poor Stars in the Galactic Halo

The Chemical Compositions of Non-Variable Field Horizontal Branch Stars -- RHB and BHB

The Chemical Compositions of Variable Field Horizontal Branch Stars -- RR Lyrae Stars

Galaxies

Spitzer Legacy program -- SAGE: Surveying the Agents of a Galaxy's Evolution

Gas and Star Formation in The Circinus Galaxy

The Magellanic System

Spectroscopic Detection of Gravity Modes in the Long-Period Pulsating Subdwarf B Star

The first spectroscopic campaign on a PG 1716 variable (long-period pulsating subdwarf B star) has successfully detected low-level velocity variations due to g-mode pulsations in PG 1627+017, superimposed on the known orbital motion. The strongest velocity mode is barely detectable at 1.0–1.5 km/s, although the target is one of the brightest (V = 12.9 mag) and largest amplitude (~0.03 mag) stars in its class. Forty nights of observations on 2 m class telescopes in Arizona, South Africa, and Australia provided 84 hr of time-series spectroscopy over a time baseline of 53 days, with typical velocity errors of 5–6 km/s . The derived radial velocity amplitude spectrum, after subtracting the orbital component, shows three potential pulsational modes 3–4 σ above the mean noise level of 0.365 km/s, at 7201.0 s (138.87 μHz), 7014.6 s (142.56 μHz), and 7037.3 s (142.10 μHz). Only one feature is statistically likely to be real, but all three are tantalizingly close to, or a 1 day alias of, the three strongest periodicities found in the concurrent photometric campaign. The velocity spectrum also shows an unexpected component at twice the orbital frequency of PG 1627+017, possibly evidence of a slightly elliptical orbit, supporting Edelmann et al.'s recent results for other short-period sdB binaries. We further attempted to detect pulsational variations in the Balmer line amplitudes. The single detected periodicity of 7209 s, although weak, is consistent with theoretical expectations as a function of wavelength, and it rules out a degree index of l = 3 or 5 for that mode. Given the extreme weakness of g-mode pulsations in PG 1716 stars, we conclude that future efforts will require larger telescopes, higher efficiency spectral monitoring over longer time baselines, improved longitude coverage, and increased radial velocity precision.

Monte Carlo Simulations of Post-Common Envelope Sdb Star Plus White Dwarf Binaries

Radial velocity surveys show that a large fraction of sdB stars are in post-common envelope binaries with orbital periods between a few hours and several days (Green et al. 1997; Morales-Rueda et al. 2003, 2004; Green et al. 2005). Such short orbital periods suggest that they must have evolved via binary mass transfer and common envelope evolution. The vast majority of sdB secondaries are not detectable using optical spectra or 2MASS fluxes, nor do they show reflection effects. Therefore, given their mass functions, nearly all of the companions must be white dwarfs. Since sdB stars are relatively bright and numerous, they constitute an extremely useful sample for studying interacting binary evolution.

We present first results from Monte Carlo simulations of short-period sdB + white dwarf binaries, with various possible distributions of the orbital separation and secondary mass. We compare the simulation results with our observed distributions and discuss the implications for the common envelope evolution.

Modeling System Parameters of KBS 13 -- a Rare Reflection Effect of sdB Binary with an M dwarf Secondary

We report preliminary VRI differential photometric and spectroscopic results for KBS 13, a recently discovered non-eclipsing sdB+dM system. Radial velocity measurements indicate an orbital period of 0.2923 +/- 0.0004 days with a semi-amplitude velocity of 22.82 +/- 0.23 km/s. This suggests the smallest secondary minimum mass yet found. We discuss the distribution of orbital periods and secondary minimum masses for other similar systems.

Spitzer Legacy program -- SAGE: Surveying the Agents of a Galaxy's Evolution

PI: Margaret Meixner (STSci), SAGE weblink

Searching for Alpha-Poor Stars in the Galactic Halo

It has been known for more than 40 years that metal-poor stars have chemical compositions that are enriched by alpha-elements (e.g., Mg, Si, S, Ca and possibly Ti). These elements are overabundant by a factor of roughly two compared with the Fe-peak elements. A few striking exceptions have been discovered in recent years, such as BD+80 245, G4-36, CS 22966-043 and HE 1424-0241. Similarly low-alpha abundance patterns are also seen in the Sagittarius dSph galaxy. However, we know almost nothing about what the true occurance frequency of these alpha-poor stars is and how they form. We conducted the first homogeneous spectroscopic survey of suspected metal poor ([Fe/H]<=-0.75) and alpha-poor ([Mg/Fe]<=+0.2) stars in the Galactic halo. We are obtaining high resolution, high signal-to-noise data with the 2dCoude spectrograph of the McDonald Observatory 2.7m telescope. We present the results of the spectroscopic survey of 25 suspected metal-poor and alpha-poor stars in the Galactic halo. The spectroscopic data have been used to derive Teff, log g, [Fe/H] metallicity, and relative abundance ratios of alpha, Fe-peak and s-process neutron-capture elements in a uniform way. We found that these metal-poor stars can be classified in 3 subgroups based on their magnesium abundance ratios: alpha-rich ( [Mg/Fe] >= +0.2), mildly alpha-poor (-0.1< [Mg/Fe] < +0.2) and extremely alpha-poor ([Mg/Fe] <= -0.1). The majority of our program stars belong to the mildly alpha-poor group which do not show other obvious abundance anomalies. In particular, these stars are not depleted in s-process neutron-capture elements in contrast to previously known extremely alpha-poor stars. We also report the preliminary attempt in calculating the magnesium production yield of type Ia and type II supernovae to match our observed magnesium abundance ratios pattern.

The Chemical Compositions of Non-Variable Field Horizontal Branch Stars

We present a new detailed abundance study of field red horizontal branch (RHB) and blue horizontal branch (BHB) non-variable stars. High resolution and high S/N echelle spectra of 11 RHB and 12 BHB were obtained with the McDonald 2.7 m telescope, and the RHB sample was augmented by reanalysis of spectra of 25 stars from a recent survey. We derived stellar atmospheric parameters based on spectroscopic constraints, and computed relative abundance ratios for 24 species of 19 elements. The species include Si II and Ca II, which have not been previously studied in RHB and BHB (Teff < 9000K) stars. The abundance ratios are generally consistent with those of similar-metallicity field stars in different evolutionary stages. We estimated the masses of the RHB and BHB stars by comparing their Teff-log g positions with HB model evolutionary tracks. The mass distribution suggests that our program stars possess masses of ~0.5 Msun, consistent with theoretical predictions. Finally, we compared the temperature distributions of field RHB, BHB stars with field RR Lyraes in the metallicity range -0.8 >~ [Fe/H] >~ -2.5. This yielded effective temperatures estimates of 5900 K and 7400 K for the red and blue fundamental edges of the RR Lyrae instability strip.

Gas and Star Formation in The Circinus Galaxy

Under Construction.

The Magellanic System