arXiv subjects

R. A. Benjamin

Publications and source records attributed to R. A. Benjamin.

22 records · Page 2Linked to original sources

arXiv2004

A GLIMPSE of Star Formation in the Giant H II Region RCW 49

GLIMPSE imaging using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope indicates that star formation is ongoing in the RCW 49 giant H II region. A photometric comparison of the sources in RCW 49 to a similar area to its north finds that at least 300 stars brighter than 13th magnitude in band [3.6] have infrared excesses inconsistent with reddening due to foreground extinction. These are likely young stellar objects (YSOs) more massive than 2.5 M_sun suggesting that thousands more low-mass stars are forming in this cloud. Some of the YSOs are massive (B stars) and therefore very young, suggesting that a new generation of star formation is occurring, possibly triggered by stellar winds and shocks generated by the older (2-3 Myr) central massive cluster. The Spitzer IRAC camera has proven to be ideally suited for distinguishing young stars from field stars, and the GLIMPSE survey of the Galactic Plane will likely find thousands of new star formation regions.

astro-ph↗

arXiv2004

Detection of a companion to the pulsating sdB Feige 48

We present the discovery of a binary companion to the pulsating sdB Feige 48. Using HST/STIS and archival FUSE spectra, we measure a period of 0.376+/-0.003 d and a velocity semi-amplitude of 28.0+/-0.2 km/s. This implies that the companion star must either be of very low mass, or the orbit is at high inclination. Combining 2MASS fluxes, the lack of a reflection effect, results from asteroseismology and a measurement of the rotation velocity of Feige 48, we show that the orbital inclination must be <= 11.4 degrees and that the unseen companion is a white dwarf with mass >= 0.46 Msun. The implications of this discovery, and of binarity amongst sdB pulsators, is then discussed in the context of recent theoretical work on sdB formation. In particular we suggest that radial velocity studies focus on sdB pulsators with no known companion, and that asteroseismological studies of sdBs investigate a larger mass range than previously considered in order to test formation models.

astro-ph↗

arXiv2003

Pulsar PSR B0656+14, the Monogem Ring, and the Origin of the `Knee' in the Primary Cosmic Ray Spectrum

The Monogem ring is a bright, diffuse, 25-degree-diameter supernova remnant easily visible in soft X-ray images of the sky. Projected within the ring is a young radio pulsar, PSR B0656+14. An association between the remnant and pulsar has been considered, but was seemingly ruled out by the direction and magnitude of the pulsar proper motion and by a distance estimate that placed the pulsar twice as far from Earth as the remnant. Here we show that in fact the pulsar was born very close to the center of the expanding remnant, both in distance and projection. The inferred pulsar and remnant ages are in good agreement. The conclusion that the pulsar and remnant were born in the same supernova explosion is nearly inescapable. The remnant distance and age are in remarkable concordance with the predictions of a model for the primary cosmic ray energy spectrum in which the `knee' feature is produced by a single dominant source.

astro-ph↗

arXiv2002

Kinematics of Diffuse Ionized Gas Halos: A Ballistic Model of Halo Rotation

To better understand diffuse ionized gas kinematics and halo rotation in spiral galaxies, we have developed a model in which clouds are ejected from the disk and follow ballistic trajectories through the halo. The behavior of clouds in this model has been investigated thoroughly through a parameter space search and a study of individual cloud orbits. Synthetic velocity profiles have been generated in z (height above the plane) from the models for the purpose of comparing with velocity centroid data from previously obtained long-slit spectra of the edge-on spirals NGC 891 (one slit) and NGC 5775 (two slits). In each case, a purely ballistic model is insufficient in explaining observed DIG kinematics. In the case of NGC 891, the observed vertical velocity gradient is not as steep as predicted by the model, possibly suggesting a source of coupling between disk and halo rotation or an outwardly directed pressure gradient. The ballistic model more successfully explains DIG kinematics observed in NGC 5775; however, it cannot explain the observed trend of high-z gas velocities nearly reaching the systemic velocity. Such behavior can be attributed to either an inwardly directed pressure gradient or a possible tidal interaction with its companion, NGC 5774. In addition, the ballistic model predicts that clouds move radially outward as they cycle through the halo. The mass and energy fluxes estimated from the model suggest this radially outward gas migration leads to a redistribution of material that may significantly affect the evolution of the ISM.

astro-ph↗