125 research outputs found
Radial Velocities of Stars in the Galactic Center
We present results from K band slit scan observations of a ~20''x20'' region
of the Galactic center (GC) in two separate epochs more than five years apart.
The high resolution (R>=14,000) observations allow the most accurate radial
velocity and acceleration measurements of the stars in the central parsec of
the Galaxy. Detected stars can be divided into three groups based on the CO
absorption band heads at ~2.2935 microns and the He I lines at ~2.0581 microns
and ~2.112, 2.113 microns: cool, narrow-line hot and broad-line hot. The radial
velocities of the cool, late-type stars have approximately a symmetrical
distribution with its center at ~-7.8(+/-10.3) km/s and a standard deviation
~113.7(+/-10.3) km/s. Although our statistics are dominated by the brightest
stars, we estimate a central black hole mass of 3.9(+/-1.1) million solar
masses, consistent with current estimates from complete orbits of individual
stars. Our surface density profile and the velocity dispersion of the late type
stars support the existence of a low density region at the Galactic center
suggested by earlier observations. Many hot, early-type stars show radial
velocity changes higher than maximum values allowed by pure circular orbital
motions around a central massive object, suggesting that the motions of these
stars greatly deviate from circular orbital motions around the Galactic center.
The correlation between the radial velocities of the early type He I stars and
their declination offsets from Sagittarius A* suggests that a systematic
rotation is present for the early-type population. No figure rotation around
the Galactic center for the late type stars is supported by the new
observations.Comment: 61 pages, 18 figures, 7 tables; accepted for publication in
Astrophysical Journa
The Double-Lined Spectrum of LBV 1806-20
Despite much theoretical and observational progress, there is no known firm
upper limit to the masses of stars. Our understanding of the interplay between
the immense radiation pressure produced by massive stars in formation and the
opacity of infalling material is subject to theoretical uncertainties, and many
observational claims of ``the most massive star'' have failed the singularity
test. LBV 1806-20 is a particularly luminous object, L~10^6 Lsun, for which
some have claimed very high mass estimates (M_initial>200 Msun), based, in
part, on its similarity to the Pistol Star. We present high-resolution
near-infrared spectroscopy of LBV 1806-20, showing that it is possibly a binary
system with components separated in velocity by ~70 kms. If correct, then this
system is not the most massive star known, yet it is a massive binary system.
We argue that a binary, or merged, system is more consistent with the ages of
nearby stars in the LBV 1806-20 cluster. In addition, we find that the velocity
of V_LSR=36 kms is consistent with a distance of 11.8 kpc, a luminosity of
10^6.3 Lsun, and a system mass of ~130 Msun.Comment: ApJL, accepte
First record of the critically endangered Hydrangea steyermarkii Standl. (Hydrangeaceae) in Mexico, and description of a new widespread Hydrangea species of Mesoamerica
Based on our detailed observations in the field, completed with herbarium material of representatives of Hydrangea section Cornidia in southeastern Mexico and Central America, we found that Hydrangea steyermarkii Standley, in contrast to what has been assumed up to now, is restricted to the Tacana Volcano on the border of Mexico and Guatemala. The majority of the specimens with reddish or whitish indument on their branches in South East Mexico and Central America, belonging to this nearly exclusively Neotropical Hydrangea clade, had been identified as such, but the characters of these collections do not coincide with the single type specimen of H. steyermarkii, neither with other individuals of this species growing in the type locality area. In fact, nearly all specimens of Mesoamerica that had been identified as this species belong to a new species with a relatively wide distribution, which we describe here. This new species is illustrated and compared with H. steyermarkii, of which we also provide an amended description and illustrations. Finally, on the basis of these two species we document previously suggested dioecism in this plant group, which not only has consequences for systematics and nomenclature in this clade, but also for conservation
The chemical abundances in the Galactic Centre from the atmospheres of Red Supergiants
The Galactic Centre (GC) has experienced a high degree of recent star-forming
activity, as evidenced by the large number of massive stars currently residing
there. The relative abundances of chemical elements in the GC may provide
insights into the origins of this activity. Here, we present high-resolution
-band spectra of two Red Supergiants in the GC (IRS~7 and VR~5-7), and in
combination with spectral synthesis we derive abundances for Fe and C, as well
as other -elements Ca, Si, Mg Ti and O. We find that the C-depletion in
VR~5-7 is consistent with the predictions of evolutionary models of RSGs, while
the heavy depletion of C and O in IRS~7's atmosphere is indicative of deep
mixing, possibly due to fast initial rotation and/or enhanced mass-loss. Our
results indicate that the {\it current} surface Fe/H content of each star is
slightly above Solar. However, comparisons to evolutionary models indicate that
the {\it initial} Fe/H ratio was likely closer to Solar, and has been driven
higher by H-depletion at the stars' surface. Overall, we find /Fe
ratios for both stars which are consistent with the thin Galactic disk. These
results are consistent with other chemical studies of the GC, given the
precision to which abundances can currently be determined. We argue that the GC
abundances are consistent with a scenario in which the recent star-forming
activity in the GC was fuelled by either material travelling down the Bar from
the inner disk, or from the winds of stars in the inner Bulge -- with no need
to invoke top-heavy stellar Initial Mass Functions to explain anomalous
abundance ratios.Comment: 23 pages, 5 figs. Accepted for publication in Ap
- …