519 research outputs found
Uncovering the Beast: Discovery of Embedded Massive Stellar Clusters in W49A
We present subarcsecond J, H, and Ks images (FWHM ~ 0.5") of an unbiased 5'x
5' (16pc x 16pc) survey of the densest region of the W49 giant molecular cloud.
The observations reveal 4 massive stellar clusters (with stars as massive as
\~120 Msun), the larger (Cluster 1) about 3 pc East of the well known Welch
ring of ultra-compact Hii regions. Cluster 1 is a) extincted by at least Av >
20 mag of foreground (unrelated and local) extinction, b) has more than 30 mag
of internal inhomogeneous visual extinction implying that it is still deeply
buried in its parental molecular cloud, and c) is powering a 6 pc diameter
giant Hii region seen both at the NIR and radio continuum. We also identify the
exciting sources of several UCHii regions. The census of massive stars in W49A
agrees or is slightly overabundant when compared with the number of Lyman
continuum photons derived from radio observations. We argue that although the
formation of the Welch ring could have been triggered by Cluster 1, the entire
W49A starburst region seems to have been multi-seeded instead of resulting from
a coherent trigger.Comment: ApJ Letters accepted. All figures provided as nice resolution
jpeg/gif files. Get full-res version at http://www.eso.org/~jalves/W49A.pd
Internal Kinematics of Luminous Compact Blue Galaxies
We describe the dynamical properties which may be inferred from HST/STIS
spectroscopic observations of luminous compact blue galaxies (LCBGs) between
0.1<z<0.7. While the sample is homogeneous in blue rest-frame color, small size
and line-width, and high surface-brightness, their detailed morphology is
eclectic. Here we determine the amplitude of rotation versus random, or
disturbed motions of the ionized gas. This information affirms the accuracy of
dynamical mass and M/L estimates from Keck integrated line-widths, and hence
also the predictions of the photometric fading of these unusual galaxies. The
resolved kinematics indicates this small subset of LCBGs are dynamically hot,
and unlikely to be embedded in disk systems.Comment: To appear in "Starbursts: from 30 Doradus to Lyman Break Galaxies"
2005, eds. R. de Grijs and R. M. Gonzalez Delgado (Kluwer
Epiphytic biomass of a tropical montane forest varies with topography
The spatial heterogeneity of tropical forest epiphytes has rarely been quantified in terms of biomass. In particular, the effect of topographic variation on epiphyte biomass is poorly known, although forests on ridges and ravines can differ drastically in stature and exposure. In an Ecuadorian lower montane forest we quantified epiphytic biomass along two gradients: (1) the twig-branch-trunk trajectory, and (2) the ridge-ravine gradient. Twenty-one trees were sampled in each of three forest types (ridge, slope, ravine positions). Their epiphytic biomass was extrapolated to stand level based on basal area-epiphyte load relationships, with tree basal areas taken from six plots of 400 m 2 each per forest type. Our results document the successional addition and partial replacement of lichens by bryophytes, angiosperms and finally dead organic matter along the twig-branch-trunk trajectory. Despite having the highest tree basal area, total epiphytic biomass (mean ± SD) of ravine forest was significantly lower (2.6 ± 0.7 Mg ha -1) than in mid-slope forest (6.3 ± 1.1 Mg ha -1) and ridge forest (4.4 ± 1.6 Mg ha -1), whereas maximum bryophyte water storage capacity was significantly higher. We attribute this pattern to differences in forest dynamics, stand structure and microclimate. Although our study could not differentiate between direct effects of slope position (nutrient availability, mesoclimate) and indirect effects (stand structure and dynamics), it provides evidence that fine-scale topography needs to be taken into account when extrapolating epiphytic biomass and related matter fluxes from stand-level data to the regional scale. © Copyright Cambridge University Press 2011
A physically motivated and empirically calibrated method to measure effective temperature, metallicity, and Ti abundance of M dwarfs
The ability to perform detailed chemical analysis of Sun-like F-, G-, and
K-type stars is a powerful tool with many applications including studying the
chemical evolution of the Galaxy and constraining planet formation theories.
Unfortunately, complications in modeling cooler stellar atmospheres hinders
similar analysis of M-dwarf stars. Empirically-calibrated methods to measure M
dwarf metallicity from moderate-resolution spectra are currently limited to
measuring overall metallicity and rely on astrophysical abundance correlations
in stellar populations. We present a new, empirical calibration of synthetic M
dwarf spectra that can be used to infer effective temperature, Fe abundance,
and Ti abundance. We obtained high-resolution (R~25,000), Y-band (~1 micron)
spectra of 29 M dwarfs with NIRSPEC on Keck II. Using the PHOENIX stellar
atmosphere modeling code (version 15.5), we generated a grid of synthetic
spectra covering a range of temperatures, metallicities, and
alpha-enhancements. From our observed and synthetic spectra, we measured the
equivalent widths of multiple Fe I and Ti I lines and a temperature-sensitive
index based on the FeH bandhead. We used abundances measured from
widely-separated solar-type companions to empirically calibrate transformations
to the observed indices and equivalent widths that force agreement with the
models. Our calibration achieves precisions in Teff, [Fe/H], and [Ti/Fe] of 60
K, 0.1 dex, and 0.05 dex, respectively and is calibrated for 3200 K < Teff <
4100 K, -0.7 < [Fe/H] < +0.3, and -0.05 < [Ti/Fe] < +0.3. This work is a step
toward detailed chemical analysis of M dwarfs at a similar precision achieved
for FGK stars.Comment: accepted for publication in ApJ, all synthetic spectra available at
http://people.bu.edu/mveyette/phoenix
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