9,138 research outputs found
The Luminous and Carbon-Rich Supernova 2006gz: A Double Degenerate Merger?
Spectra and light curves of SN 2006gz show the strongest signature of
unburned carbon and one of the slowest fading light curves ever seen in a type
Ia event (Delta m_15 = 0.69 +/- 0.04). The early-time Si II velocity is low,
implying it was slowed by an envelope of unburned material. Our best estimate
of the luminosity implies M_V = -19.74 and the production of ~ 1.2 M_sun of
56Ni. This suggests a super-Chandrasekhar mass progenitor. A double degenerate
merger is consistent with these observations.Comment: Accepted for publication in ApJL (5 pages, 4 figures). UBVr'i' light
curves, UVOIR light curves, and spectra available at
http://www.cfa.harvard.edu/supernova/SN2006g
Discovery of hard X-ray features around hotspots of Cygnus A
We present results of analysis of a Chandra observation of Cygnus A in which
the X-ray hotspots at the ends of the jets are mapped in detail. A hardness map
reveals previously unknown structure in the form of outer and inner hard arcs
around the hotspots, with hardness significantly enhanced compared with the
hotspot central regions. The outer hard arcs may constitute the first detection
of the bow shock; the inner hard arcs may reveal where the jets impact on the
hotspots. We argue that these features cannot result from electrons radiating
by the synchrotron self-Compton process. Instead we consider two possible
sources of the hard emission: the outer arcs may be due to thermal radiation of
hot intracluster gas compressed at the bow shock. Alternatively, both outer and
inner arcs may be due to synchrotron radiation of electrons accelerated in
turbulent regions highly perturbed by shocks and shear flows. Comparison of
measured hardness ratios with simulations of the hardness ratios resulting from
these processes show that it is more diffcult to explain the observations with
a thermal model. Although we cannot rule out a thermal model, we argue in
favour of the non-thermal explanation. The hard regions in the secondary
hotspots suggest that jet activity is still powering these hotspots.Comment: MNRAS in press; 5 pages, 3 figures (2 figures in colour in jpeg
format should be printed separately
Stability of strained heteroepitaxial systems in (1+1) dimensions
We present a simple analytical model for the determination of the stable
phases of strained heteroepitaxial systems in (1+1) dimensions. In order for
this model to be consistent with a subsequent dynamic treatment, all
expressions are adjusted to an atomistic Lennard-Jones system. Good agreement
is obtained when the total energy is assumed to consist of two contributions:
the surface energy and the elastic energy. As a result, we determine the stable
phases as a function of the main ``control parameters'' (binding energies,
coverage and lattice mismatch). We find that there exists no set of parameters
leading to an array of islands as a stable configuration. We however show that
a slight modification of the model can lead to the formation of stable arrays
of islands.Comment: 11 pages, 14 figures, submitted to Physical Review
Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, Galactic chemical evolution, and exoplanets
We derive atmospheric parameters and lithium abundances for 671 stars and
include our measurements in a literature compilation of 1381 dwarf and subgiant
stars. First, a "lithium desert" in the effective temperature (Teff) versus
lithium abundance (A_Li) plane is observed such that no stars with Teff~6075 K
and A_Li~1.8 are found. We speculate that most of the stars on the low A_Li
side of the desert have experienced a short-lived period of severe surface
lithium destruction as main-sequence or subgiant stars. Next, we search for
differences in the lithium content of thin-disk and thick-disk stars, but we
find that internal processes have erased from the stellar photospheres their
possibly different histories of lithium enrichment. Nevertheless, we note that
the maximum lithium abundance of thick-disk stars is nearly constant from
[Fe/H]=-1.0 to -0.1, at a value that is similar to that measured in very
metal-poor halo stars (A_Li~2.2). Finally, differences in the lithium abundance
distribution of known planet-host stars relative to otherwise ordinary stars
appear when restricting the samples to narrow ranges of Teff or mass, but they
are fully explained by age and metallicity biases. We confirm the lack of a
connection between low lithium abundance and planets. However, we find that no
low A_Li planet-hosts are found in the desert Teff window. Provided that subtle
sample biases are not responsible for this observation, this suggests that the
presence of gas giant planets inhibit the mechanism responsible for the lithium
desert.Comment: ApJ, in press. Complete Tables 1 and 3 are available upon reques
Optical Surface Photometry of a Sample of Disk Galaxies. II Structural Components
This work presents the structural decomposition of a sample of 11 disk
galaxies, which span a range of different morphological types. The U, B, V, R,
and I photometric information given in Paper I (color and color-index images
and luminosity, ellipticity, and position-angle profiles) has been used to
decide what types of components form the galaxies before carrying out the
decomposition. We find and model such components as bulges, disks, bars, lenses
and rings.Comment: 14 figures. Accepted for publication in A&
The abundance of 28Si32S, 29Si32S, 28Si34S, and 30Si32S in the inner layers of the envelope of IRC+10216
We present high spectral resolution mid-IR observations of SiS towards the
C-rich AGB star IRC+10216 carried out with the Texas Echelon-cross-Echelle
Spectrograph mounted on the NASA Infrared Telescope Facility. We have
identified 204 ro-vibrational lines of 28Si32S, 26 of 29Si32S, 20 of 28Si34S,
and 15 of 30Si32S in the frequency range 720-790 cm-1. These lines belong to
bands v=1-0, 2-1, 3-2, 4-3, and 5-4, and involve rotational levels with
Jlow<90. About 30 per cent of these lines are unblended or weakly blended and
can be partially or entirely fitted with a code developed to model the mid-IR
emission of a spherically symmetric circumstellar envelope composed of
expanding gas and dust. The observed lines trace the envelope at distances to
the star <35R* (~0.7 arcsec). The fits are compatible with an expansion
velocity of 1+2.5(r/R*-1) km/s between 1 and 5R*, 11 km/s between 5 and 20R*,
and 14.5 km/s outwards. The derived abundance profile of 28Si32S with respect
to H2 is 4.9e-6 between the stellar photosphere and 5R*, decreasing linearly to
1.6e-6 at 20R* and to 1.3e-6 at 50R*. 28Si32S seems to be rotationally under
LTE in the region of the envelope probed with our observations and
vibrationally out of LTE in most of it. There is a red-shifted emission excess
in the 28Si32S lines of band v=1-0 that cannot be found in the lines of bands
v=2-1, 3-2, 4-3, and 5-4. This excess could be explained by an enhancement of
the vibrational temperature around 20R* behind the star. The derived isotopic
ratios 28Si/29Si, and 32S/34S are 17 and 14, compatible with previous
estimates.Comment: 11 pages, 5 figures, and 4 tables. Accepted for publication in MNRA
The Infrared Massive Stellar Content of M83
We present an analysis of archival Spitzer images and new ground-based and
Hubble Space Telescope (HST) near-infrared (IR) and optical images of the field
of M83 with the goal of identifying rare, dusty, evolved massive stars. We
present point source catalogs consisting of 3778 objects from
Infrared Array Camera (IRAC) Band 1 (3.6 m) and Band 2 (4.5 m), and
975 objects identified in Magellan 6.5m FourStar near-IR and
images. A combined catalog of coordinate matched near- and mid-IR point sources
yields 221 objects in the field of M83. Using this photometry we identify 185
massive evolved stellar candidates based on their location in color-magnitude
and color-color diagrams. We estimate the background contamination to our
stellar candidate lists and further classify candidates based on their
appearance in Wide Field Camera 3 (WFC3) observations of M83. We find 49
strong candidates for massive stars which are very promising objects for
spectroscopic follow-up. Based on their location in a versus
diagram, we expect at least 24, or roughly 50%, to be confirmed as red
supergiants.Comment: 32 pages, 23 figures, accepted for publication in A&
Accurate fundamental parameters and distance to a massive early-type eclipsing binary in the Danks 2 cluster
We present a study of the properties of the O-type, massive eclipsing binary
2MASS J13130841-6239275 located in the outskirts of the Danks 2 cluster in the
G305 star-forming complex, using near-infrared spectroscopy from VLT/ISAAC. We
derive the masses and radii to be 24.50.9 M and 9.20.1
R for the primary and 21.70.8 M and 8.70.1
R for the secondary component. In addition, we evaluate the
sensitivity of our parameters to the choice of the spectral features used to
determine the radial velocities. Both components appear to be main-sequence
O6.5O7 type stars at an age of 5 Myr, which is in agreement with the
age of the cluster. A high visual extinction of A=11.90.1 mag is
reported, which is likely attributed to the cold molecular gas contaminating
the north-east region of the cluster. By fitting the spectral energy
distribution of the system to the available photometry, we
determine a distance to the system of 3.520.08 kpc with a precision of
2, which is the most well-determined distance to the Danks 2 cluster and
the host complex reported in the literature.Comment: 13 pages, 9 figures, 6 tables. Accepted for publication in A&
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