Constraining the Variability and Binary Fraction of Galactic Center Young Stars

We present constraints on the variability and binarity of young stars in the central 10 arcseconds (~0.4 pc) of the Milky Way Galactic Center (GC) using Keck Adaptive Optics data over a 12 year baseline. Given our experiment's photometric uncertainties, at least 36% of our sample's known early-type stars are variable. We identified eclipsing binary systems by searching for periodic variability. In our sample of spectroscopically confirmed and likely early-type stars, we detected the two previously discovered GC eclipsing binary systems. We derived the likely binary fraction of main sequence, early-type stars at the GC via Monte Carlo simulations of eclipsing binary systems, and find that it is at least 32% with 90% confidence.Comment: Accepted for publication in Proceedings of IAU Symposium 322: The Multi-Messenger Astrophysics of the Galactic Centre, 2 pages, 1 figur

Keck Adaptive Optics Observations of the Protostellar Disk around Radio Source I in the Orion Kleinmann-Low Nebula

We have made the first detection of a near-infrared counterpart associated with the disk around Radio Source "I," a massive protostar in the Kleinmann-Low Nebula in Orion using imaging with laser guide star adaptive optics on the Keck II telescope. The infrared emission is evident in images acquired using L' (3.8 microns) and Ms (4.7 microns) filters and is not detectable at K' (2.1 microns). The observed morphology strongly suggests that we are seeing some combination of scattered and thermal light emanating from the disk. The disk is also manifest in the L'/Ms flux ratio image. We interpret the near-infrared emission as the illuminated surface of a nearly edge-on disk, oriented so that only the northern face is visible; the opposite surface remains hidden by the disk. We do not see infrared radiation associated directly with the star proposed to be associated with Source "I." The data also suggest that there is a cavity above and below the disk that is oriented perpendicular to the disk, and is sculpted by the known, strong outflow from the inner disk of Source I. We compare our data to models of a protostar with a surrounding disk, envelope, and wind-blown cavity in order to elucidate the nature of the disk around Radio Source I.Comment: 22 pages, 7 figures. Accepted for publication to Ap

Benefits of adding fluticasone propionate/salmeterol to tiotropium in moderate to severe COPD

SummaryBackgroundCombining maintenance medications with different mechanisms of action may improve outcomes in COPD. In this study we evaluated the efficacy and safety of fluticasone/salmeterol (FSC) (250/50 mcg twice daily) when added to tiotropium (18 mcg once daily) (TIO) in subjects with symptomatic moderate to severe COPD.MethodsThis was a 24-week, randomized, double-blind, parallel group, multi-center study. Subjects 40 years or older with cigarette smoking history ≥10 pack-years and with the diagnosis of COPD and post-bronchodilator FEV1 ≥40 to ≤ 80% of predicted normal and FEV1/FVC of ≤0.70 were enrolled. Following a 4-week treatment with open-label TIO 18 mcg once daily, subjects were randomized in a double-blind fashion to either the addition of FSC 250/50 DISKUS twice daily or matching placebo. The primary efficacy endpoint was AM pre-dose FEV1 and secondary endpoints included other measures of lung function, rescue albuterol use, health status and exacerbations.ResultsThe addition of FSC to TIO significantly improved lung function indices including AM pre-dose FEV1, 2 h post-dose FEV1, AM pre-dose FVC, 2 h post-dose FVC and AM pre-dose IC compared with TIO alone. Furthermore, this combination was superior to TIO alone in reducing rescue albuterol use. However, there were no significant differences between the treatment groups in health status or COPD exacerbations. The incidence of adverse events was similar in both groups.ConclusionsThe addition of FSC to subjects with COPD treated with TIO significantly improves lung function without increasing the risk of adverse events. NCT00784550

The Post-Periapse Evolution of Galactic Center Source G1: The second case of a resolved tidal interaction with a supermassive black hole

We present new Adaptive Optics (AO) imaging and spectroscopic measurements of Galactic Center source G1 from W. M. Keck Observatory. Our goal is to understand its nature and relationship to G2, which is the first example of a spatially-resolved object interacting with the supermassive black hole (SMBH). Both objects have been monitored with AO for the past decade (2003 - 2014) and are comparatively close to the black hole ($a_{\rm{min}} \sim$200-300 AU) on very eccentric orbits ($e_{\rm{G1}}\sim$0.99; $e_{\rm{G2}}\sim$0.96). While G2 has been tracked before and during periapse passage ($T_{0} \sim$ 2014.2), G1 has been followed since soon after emerging from periapse ($T_{0} \sim$ 2001.3). Our observations of G1 double the previously reported observational time baseline, which improves its orbital parameter determinations. G1's orbital trajectory appears to be in the same plane as that of G2, but with a significantly different argument of periapse ($\Delta\omega$ = 21$\pm$4 degrees). This suggests that G1 is an independent object and not part of a gas stream containing G2 as has been proposed. Furthermore, we show for the first time that: (1) G1 is extended in the epochs closest to periapse along the direction of orbital motion and (2) G1 becomes significantly smaller over time, (450 AU in 2004 to less than 170 AU in 2009). Based on these observations, G1 appears to be the second example of an object tidally interacting with a SMBH. G1's existence 14 years after periapse, along with its compactness in epochs further from the time of periapse, suggest that this source is stellar in nature.Comment: submitted to Ap

Detection of Galactic Center source G2 at 3.8 μ\mum during periapse passage

We report new observations of the Galactic Center source G2 from the W. M. Keck Observatory. G2 is a dusty red object associated with gas that shows tidal interactions as it nears closest approach with the Galaxy's central black hole. Our observations, conducted as G2 passed through periapse, were designed to test the proposal that G2 is a 3 earth mass gas cloud. Such a cloud should be tidally disrupted during periapse passage. The data were obtained using the Keck II laser guide star adaptive optics system (LGSAO) and the facility near-infrared camera (NIRC2) through the K' [2.1 $\mu$m] and L' [3.8 $\mu$m] broadband filters. Several results emerge from these observations: 1) G2 has survived its closest approach to the black hole as a compact, unresolved source at L'; 2) G2's L' brightness measurements are consistent with those over the last decade; 3) G2's motion continues to be consistent with a Keplerian model. These results rule out G2 as a pure gas cloud and imply that G2 has a central star. This star has a luminosity of $\sim$30 $L_{\odot}$ and is surrounded by a large ($\sim$2.6 AU) optically thick dust shell. The differences between the L' and Br-$\gamma$ observations can be understood with a model in which L' and Br-$\gamma$ emission arises primarily from internal and external heating, respectively. We suggest that G2 is a binary star merger product and will ultimately appear similar to the B-stars that are tightly clustered around the black hole (the so-called S-star cluster).Comment: Accepted by ApJ Letters, 2014 October 1

High Spectral Resolution Observations of the Massive Stars in the Galactic Center

We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25 arcseconds of the Galactic center. We have identified a total of twenty-one emission-line stars, seven of which are new radial velocity detections with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He I doublet absorption lines, and 2) those with broad 2.058 micron He I emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 kms, smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the He I absorption doublet are more robust than those previously estimated from the 2.058 micron emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multi-epoch radial velocity measurements for IRS 16NE show a change in its velocity presumably due to an unseen stellar companion.Comment: ApJ accepted, 42 pages, 16 figure

Contagion dynamics in time-varying metapopulation networks

The metapopulation framework is adopted in a wide array of disciplines to describe systems of well separated yet connected subpopulations. The subgroups or patches are often represented as nodes in a network whose links represent the migration routes among them. The connections have been so far mostly considered as static, but in general evolve in time. Here we address this case by investigating simple contagion processes on time-varying metapopulation networks. We focus on the SIR process and determine analytically the mobility threshold for the onset of an epidemic spreading in the framework of activity-driven network models. We find profound differences from the case of static networks. The threshold is entirely described by the dynamical parameters defining the average number of instantaneously migrating individuals and does not depend on the properties of the static network representation. Remarkably, the diffusion and contagion processes are slower in time-varying graphs than in their aggregated static counterparts, the mobility threshold being even two orders of magnitude larger in the first case. The presented results confirm the importance of considering the time-varying nature of complex networks

A population of dust-enshrouded objects orbiting the Galactic black hole

The central 0.1 parsecs of the Milky Way host a supermassive black hole identified with the position of the radio and infrared source Sagittarius A*, a cluster of young, massive stars (the S stars) and various gaseous features. Recently, two unusual objects have been found to be closely orbiting Sagittarius A*: the so-called G sources, G1 and G2. These objects are unresolved (having a size of the order of 100 astronomical units, except at periapse, where the tidal interaction with the black hole stretches them along the orbit) and they show both thermal dust emission and line emission from ionized gas. G1 and G2 have generated attention because they appear to be tidally interacting with the supermassive Galactic black hole, possibly enhancing its accretion activity. No broad consensus has yet been reached concerning their nature: the G objects show the characteristics of gas and dust clouds but display the dynamical properties of stellar-mass objects. Here we report observations of four additional G objects, all lying within 0.04 parsecs of the black hole and forming a class that is probably unique to this environment. The widely varying orbits derived for the six G objects demonstrate that they were commonly but separately formed

High Precision Stellar Radial Velocities in the Galactic Center

We present radial velocities for 85 cool stars projected onto the central parsec of the Galaxy. The majority of these velocities have relative errors of $\sim$1 km/s, or a factor of $\sim$30-100 smaller than those previously obtained with proper motion or other radial velocity measurements for a similar stellar sample. The error in a typical individual stellar velocity, including all sources of uncertainty, is 1.7 km/s. Two similar data sets were obtained one month apart, and the total error in the relative velocities is 0.80 km/s\ in the case where an object is common to both data sets. The data are used to characterize the velocity distribution of the old population in the Galctic Center. We find that the stars have a Gaussian velocity distribution with a mean heliocentric velocity of $-10.1\pm$11.0 km/s (blueshifted) and a standard deviation of 100.9$\pm7.7$ km/s; the mean velocity of the sample is consistent with no bulk line-of-sight motion with respect to the Local Standard of Rest. At the 1 sigma level, the data are consistent with a symmetric velocity distribution about any arbitrary axis in the plane of the sky. We find evidence for a flattening in the distribution of late-type stars within a radius of $\sim$0.4 \pc, and infer a volume density distribution of r$^{-1/4}$ in this region. Finally, we establish a first epoch of radial velocity measurements which can be compared to subsequent epochs to measure small accelerations (1 km/s/yr), corresponding to the magnitude expected over a timespan of several years for stars nearest to Sgr A*.Comment: retrieve full version at http://www-int.stsci.edu/$\sim$figer/papers/nirspec/vel