The Galactic Center: Not an Active Galactic Nucleus

We present 10um-35um Spitzer spectra of the interstellar medium in the Central Molecular Zone (CMZ), the central 210 pc x 60 pc of the Galactic center (GC). We present maps of the CMZ in ionic and H2 emission, covering a more extensive area than earlier spectroscopic surveys in this region. The radial velocities and intensities of ionic lines and H2 suggest that most of the H2 0-0 S(0) emission comes from gas along the line-of-sight, as found by previous work. We compare diagnostic line ratios measured in the Spitzer Infrared Nearby Galaxies Survey (SINGS) to our data. Previous work shows that forbidden line ratios can distinguish star-forming galaxies from LINERs and AGNs. Our GC line ratios agree with star-forming galaxies and not with LINERs or AGNs.Comment: Accepted for publication in the ApJ Supplement Serie

The Dwarf Galaxy NGC 1705 -- A Highly Composite Stellar Population

We present visible and near-infrared images of the amorphous dwarf galaxy NGC 1705 in the $B$, $V$, $R$, $J$, $H$, and $K$ bands. Optical and near infrared colors in the galaxy are consistent with a composite population with colors similar to those of the bluest Sc nuclei. The contribution of young stars to the disk population of the galaxy decreases outwards from the center. In the near infrared images we detect three unresolved point sources, two of which have colors and luminosities comparable to intermediate age LMC clusters.Comment: uuencoded compressed tarred postscript including figures. Accepted for publication in AJ

Metallicity of Red Giants in the Galactic Bulge from Near-Infrared Spectroscopy

We present K-band spectra of more than 110 M giants in Galactic bulge fields interior to -4 degrees and as close as 0.2 degrees of the Galactic Center. From the equivalent widths of three features in these spectra, EW(Na),EW(Ca), and EW(CO) we calculate [Fe/H] for the stars with a calibration derived from globular clusters Stephens et al (2000). The mean [Fe/H] for each field is in good agreement with the results from Frogel et al. (1999) based on the slope of the giant branch method. We find no evidence for a metallicity gradient along the minor or major axes of the inner bulge (R < 0.6 kpc). A metallicity gradient along the minor axis, found earlier, arises when fields located at larger galactic radius are included. However, these more distant fields are located outside of the infrared bulge defined by the COBE/DIRBE observations. We compute the [Fe/H] distribution for the inner bulge and find a mean value of -0.21 dex with a full width dispersion of 0.30 dex, close to the values found for Baade's Window (BW) by Sadler et al. (1996) and to a theoretical prediction for a bulge formed by dissipative collapse Molla et al (2000).Comment: 32 pages, 10 figures, AJ submitte

Chemical Abundances in the Galactic Center

We present chemical abundances in a sample of luminous cool stars located within 50 pc of the Galactic Center. Abundances of C, N, O, Ca and Fe are derived using high-resolution infrared spectra in the H- and K- bands. We report solar iron abundance, enhanced alpha element abundances, and CN-cycle mixed material in the atmospheres of these evolved stars

Abundant Methanol Ice toward a Massive Young Stellar Object in the Central Molecular Zone

Previous radio observations revealed widespread gas-phase methanol (CH_3OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH_3OH ice toward a star in the CMZ, based on a Subaru 3.4–4.0 μm spectrum, aided by NASA/IRTF L’ imaging and 2–4 μm spectra. The star lies ~8000 au away in projection from a massive young stellar object (MYSO). Its observed high CH_3OH ice abundance (17% ± 3% relative to H_2O ice) suggests that the 3.535 μm CH_3OH ice absorption likely arises in the MYSO's extended envelope. However, it is also possible that CH_3OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH_3OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH_3OH abundance confirms the prominent 15.4 μm shoulder absorption observed toward GC MYSOs arises from CO_2 ice mixed with CH_3OH

Characterizing the Variability of Stars with Early-release Kepler Data

We present a variability analysis of the early-release first quarter of data publicly released by the Kepler project. Using the stellar parameters from the Kepler Input Catalog, we have separated the sample into 129,000 dwarfs and 17,000 giants and further sub-divided the luminosity classes into temperature bins corresponding approximately to the spectral classes A, F, G, K, and M. Utilizing the inherent sampling and time baseline of the public data set (30 minute sampling and 33.5 day baseline), we have explored the variability of the stellar sample. The overall variability rate of the dwarfs is 25% for the entire sample, but can reach 100% for the brightest groups of stars in the sample. G dwarfs are found to be the most stable with a dispersion floor of σ ~ 0.04 mmag. At the precision of Kepler, >95% of the giant stars are variable with a noise floor of ~0.1 mmag, 0.3 mmag, and 10 mmag for the G giants, K giants, and M giants, respectively. The photometric dispersion of the giants is consistent with acoustic variations of the photosphere; the photometrically derived predicted radial velocity distribution for the K giants is in agreement with the measured radial velocity distribution. We have also briefly explored the variability fraction as a function of data set baseline (1-33 days), at the native 30 minute sampling of the public Kepler data. To within the limitations of the data, we find that the overall variability fractions increase as the data set baseline is increased from 1 day to 33 days, in particular for the most variable stars. The lower mass M dwarf, K dwarf, and G dwarf stars increase their variability more significantly than the higher mass F dwarf and A dwarf stars as the time baseline is increased, indicating that the variability of the lower mass stars is mostly characterized by timescales of weeks while the variability of the higher mass stars is mostly characterized by timescales of days. A study of the distribution of the variability as a function of galactic latitude suggests that sources closer to the galactic plane are more variable. This may be the result of sampling differing populations (i.e., ages) as a function of latitude or may be the result of higher background contamination that is inflating the variability fractions at lower latitudes. A comparison of the M dwarf statistics to the variability of 29 known bright M dwarfs indicates that the M dwarfs are primarily variable on timescales of weeks or longer presumably dominated by spots and binarity. On shorter timescales of hours, which are relevant for planetary transit detection, the stars are significantly less variable, with ~80% having 12 hr dispersions of 0.5 mmag or less

Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes

To study the demographics of interstellar ices in the Central Molecular Zone (CMZ) of the Milky Way, we obtain near-infrared spectra of $109$ red point sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and mid-infrared photometry, including $12$ objects in the previous paper of this series, to ensure that these sources trace a large amount of absorption through clouds in each line of sight. We find that most of the sample ($100$ objects) show CO band-head absorption at $2.3\ \mu$m, tagging them as red (super-) giants. Despite the photospheric signature, however, a fraction of the sample with $L$-band spectra ($9/82=0.11$) exhibit large H$_2$O ice column densities ($N > 2\times10^{18}\ {\rm cm}^{-2}$), and six of them also reveal CH$_3$OH ice absorption. As one of such objects is identified as a young stellar object (YSO) in our previous work, these ice-rich sight lines are likely associated with background stars in projection to an extended envelope of a YSO or a dense cloud core. The low frequency of such objects in the early stage of stellar evolution implies a low star-formation rate ($<0.02\ M_\odot$ yr$^{-1}$), reinforcing the previous claim on the suppressed star-formation activity in the CMZ. Our data also indicate that the strong "shoulder" CO$_2$ ice absorption at $15.4\ \mu$m observed in YSO candidates in the previous paper arises from CH$_3$OH-rich ice grains having a large CO$_2$ concentration [$N {\rm (CO_2)} / N {\rm (CH_3OH)} \approx 1/3$].Comment: 28 pages, 12 figures, 3 tables. Accepted for publication in the Astrophysical Journa