11,757 research outputs found
Analysis of F and G Subdwarfs. I. The Location of Subdwarfs in the Theoretical H-R Diagram
Spectrum scans and model stellar atmospheres have been used to locate in the (L/L_⊙, T_(eff)) plane F and G subdwarfs having known parallax A comparison is made between their position on the H-R diagram and the evolutionary tracks for Population II models of low- and high-helium content recently computed by Faulkner and Iben. The lower helium-content models are found to give somewhat more plausible ages than the high-helium models. Nevertheless, for both the low- and high-helium-content models, we find several subdwarfs with ages apparently greater than 10 X 10^9 years
An ABC transporter containing a forkhead-associated domain interacts with a serine-threonine protein kinase and is required for growth of Mycobacterium tuberculosis in mice
Forkhead-associated (FHA) domains are modular phosphopeptide recognition motifs with a striking preference for phosphothreonine-containing epitopes. FHA domains have been best characterized in eukaryotic signaling pathways but have been identified in six proteins in Mycobacterium tuberculosis, the causative organism of tuberculosis. One of these, coded by gene Rv1747, is an ABC transporter and the only one to contain two such modules. A deletion mutant of Rv1747 is attenuated in a mouse intravenous injection model of tuberculosis where the bacterial load of the mutant is 10-fold lower than that of the wild type in both lungs and spleen. In addition, growth of the mutant in mouse bone marrow-derived macrophages and dendritic cells is significantly impaired. In contrast, growth of this mutant in vitro was indistinguishable from that of the wild type. The mutant phenotype was lost when the mutation was complemented by the wild-type allele, confirming that it was due to mutation of Rv1747. Using yeast two-hybrid analysis, we have shown that the Rv1747 protein interacts with the serine-threonine protein kinase PknF. This interaction appears to be phospho-dependent since it is abrogated in a kinase-dead mutant and by mutations in the presumed activation loop of PknF and in the first FHA domain of Rv1747. These results demonstrate that the protein coded by Rv1747 is required for normal virulent infection by M. tuberculosis in mice and, since it interacts with a serine-threonine protein kinase in a kinase-dependent manner, indicate that it forms part of an important phospho-dependent signaling pathway
Oscillation Effects On Neutrinos From The Early Phase Of a Nearby Supernova
Neutrinos emitted during stellar core collapse leading to a supernova are
primarily of the electron neutrino type at source which may undergo oscillation
between flavor eigenstates during propagation to an earth-bound detector.
Although the number of neutrinos emitted during the pre-bounce collapse phase
is much smaller than that emitted in the post-bounce phase (in which all
flavors of neutrinos are emitted), a nearby supernova event may nevertheless
register a substantial number of detections from the pre-bounce phase at
SuperKamiokande (SK) and the Sudbury Neutrino Observatory (SNO). The
calorimetric measurement of the supernova neutrino fluence from this stage via
the charge current and neutral current detection channels in SNO and the
corresponding distortion of detected spectrum in SK over the no-oscillation
spectrum, can probe information about neutrino mass difference and mixing which
are illustrated here in terms of two- and three-flavor oscillation models
Modes of Multiple Star Formation
This paper argues that star forming environments should be classified into
finer divisions than the traditional isolated and clustered modes. Using the
observed set of galactic open clusters and theoretical considerations regarding
cluster formation, we estimate the fraction of star formation that takes place
within clusters. We find that less than 10% of the stellar population
originates from star forming regions destined to become open clusters,
confirming earlier estimates. The smallest clusters included in the
observational surveys (having at least N=100 members) roughly coincide with the
smallest stellar systems that are expected to evolve as clusters in a dynamical
sense. We show that stellar systems with too few members N < N_\star have
dynamical relaxation times that are shorter than their formation times (1-2
Myr), where the critical number of stars N_\star \approx 100. Our results
suggest that star formation can be characterized by (at least) three principal
modes: I. isolated singles and binaries, II. groups (N<N_\star), and III.
clusters (N>N_\star). Many -- if not most -- stars form through the
intermediate mode in stellar groups with 10<N<100. Such groups evolve and
disperse much more rapidly than open clusters; groups also have a low
probability of containing massive stars and are unaffected by supernovae and
intense ultraviolet radiation fields. Because of their short lifetimes and
small stellar membership, groups have relatively little effect on the star
formation process (on average) compared to larger open clusters.Comment: accepted to The Astrophysical Journa
Mid-infrared imaging of the young binary star Hen 3-600: Evidence for a dust disk around the primary
We present high-resolution mid-infrared observations of the nearby late-type
young binary system Hen 3-600. The binary, at a distance of 50 pc, could
be a member of the TW Hydrae Association, the nearest known group of young
stars, with an age of a few million years. Our images make it possible for the
first time to determine which star in the pair, separated by 1.4'', harbors the
mid-infrared excess detected by IRAS. In the near-infrared, where the radiation
is primarily photospheric, Hen 3-600A (M3) and Hen 3-600B (M3.5) have a flux
ratio of 1.6. At 4.8m, 10.8m, and 18.2m, the primary becomes
increasingly dominant over the secondary, suggesting that most of the
circumstellar dust in the system resides around Hen 3-600A. Comparison of the
spectral energy distribution (SED) of Hen 3-600A to the median SED of classical
T Tauri stars suggests that its disk may be truncated by the secondary and
provides tentative evidence for a central disk hole. The distribution of dust
in the Hen 3-600 system may provide important clues to the formation and
evolution of protoplanetary disks in close binaries.Comment: 9 pages, 2 PostScript figures, accepted for publication in The
Astrophysical Journal Letter
Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE
We present initial results of a deep near-IR spectroscopic survey covering
the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the
Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0< z <
2.6, star-formation rates 2 < SFR < 200 M_sun/yr, and stellar masses 8.6 <
log(M*/M_sun) < 11.4, with high-quality spectra in both H- and K-band
atmospheric windows. We show unambiguously that the locus of z~2.3 galaxies in
the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight,
relationship between the ratios [NII]6585/Halpha and [OIII]/Hbeta as compared
to local galaxies. Using photoionization models, we argue that the offset of
the z~2.3 locus relative to z~ 0 is explained by a combination of harder
ionizing radiation field, higher ionization parameter, and higher N/O at a
given O/H than applies to most local galaxies, and that the position of a
galaxy along the z~2.3 star-forming BPT locus is surprisingly insensitive to
gas-phase oxygen abundance. The observed nebular emission line ratios are most
easily reproduced by models in which the net ionizing radiation field resembles
a blackbody with effective temperature T_eff = 50000-60000 K and N/O close to
the solar value at all O/H. We critically assess the applicability of
commonly-used strong line indices for estimating gas-phase metallicities, and
consider the implications of the small intrinsic scatter in the empirical
relationship between excitation-sensitive line indices and stellar mass (i.e.,
the "mass-metallicity" relation), at z~2.3.Comment: 41 pages, 25 figures, accepted for publication in the Astrophysical
Journal. Version with full-resolution figures available at
http://www.astro.caltech.edu/~ccs/mos_bpt_submit.pd
Analysis of F and G Subdwarfs. I. The Location of Subdwarfs in the Theoretical H-R Diagram
Spectrum scans and model stellar atmospheres have been used to locate in the (L/L_⊙, T_(eff)) plane F and G subdwarfs having known parallax A comparison is made between their position on the H-R diagram and the evolutionary tracks for Population II models of low- and high-helium content recently computed by Faulkner and Iben. The lower helium-content models are found to give somewhat more plausible ages than the high-helium models. Nevertheless, for both the low- and high-helium-content models, we find several subdwarfs with ages apparently greater than 10 X 10^9 years
Massive star formation in NGC 6946
H-alpha measurements of the late-type spiral galaxy NGC 6946 are presented. Assuming an initial mass function, the H-alpha data are used to calculate the total star formation rate and efficiency of massive star formation as a function of galactocentric radius. An attempt is made to determine the factors influencing these quantities by comparing them to the surface densities of H I and H 2 and the degree of compression suffered by the gas in spiral density wave theory. The rate of star formation is very closely correlated with the first power of the surface density of H2, and somewhat less correlated with the degree of compression of the gas predicted by density wave theory. In contrast, the efficiency of massive star formation is roughly constant across the face of the galaxy
New Herbig-Haro Objects and Giant Outflows in Orion
We present the results of a photographic and CCD imaging survey for
Herbig-Haro (HH) objects in the L1630 and L1641 giant molecular clouds in
Orion. The new HH flows were initially identified from a deep H-alpha film from
the recently commissioned AAO/UKST H-alpha Survey of the southern sky. Our
scanned H-alpha and broad band R images highlight both the improved resolution
of the H-alpha survey and the excellent contrast of the H-alpha flux with
respect to the broad band R. Comparative IVN survey images allow us to
distinguish between emission and reflection nebulosity. Our CCD H-alpha, [SII],
continuum and I band images confirm the presence of a parsec-scale HH flow
associated with the Ori I-2 cometary globule and several parsec-scale strings
of HH emission centred on the L1641-N infrared cluster. Several smaller
outflows display one-sided jets. Our results indicate that for declinations
south of -6 degrees in L1641, parsec-scale flows appear to be the major force
in the large-scale movement of optical dust and molecular gas.Comment: 14 pages, Latex using MN style, 21 figures in JPEG format. Higher
resolution figures available from S.L. Mader. Accepted by MNRAS. Email
contact for higher resolution images: [email protected]
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