186 research outputs found
Supernova 1996L: evidence of a strong wind episode before the explosion
Observations of the type II SN 1996L reveal the presence of a slowly
expanding (V~700$ km/s) shell at ~ 10^(16) cm from the exploding star. Narrow
emission features are visible in the early spectra superposed on the normal SN
spectrum. Within about two months these features develop narrow symmetric
P-Cygni profiles. About 100 days after the explosion the light curve suddenly
flattens, the spectral lines broaden and the Halpha flux becomes larger than
what is expected from a purely radioactive model. These events are interpreted
as signatures of the onset of the interaction between the fast moving ejecta
and a slowly moving outer shell of matter ejected before the SN explosion. At
about 300 days the narrow lines disappear and the flux drops until the SN fades
away, suggesting that the interaction phase is over and that the shell has been
swept away. Simple calculations show that the superwind episode started 9 yr
before the SN explosion and lasted 6 yr, with an average dM/dt=10^(-3)
M_solar/yr. Even at very late epochs (up to day 335) the typical forbidden
lines of [OI], CaII], [FeII] remain undetected or very weak. Spectra after day
270 show relatively strong emission lines of HeI. These lines are narrower than
other emission lines coming from the SN ejecta, but broader than those from the
CSM. These high excitation lines are probably the result of non-thermal
excitation and ionization caused by the deposition of the gamma-rays emitted in
the decay of radioactive material mixed in the He layer.Comment: 8 pages, 6 figures, Latex, To appear in M.N.R.A.
Helium Emission in the Type Ic SN 1999cq
We present the first unambiguous detection of helium emission lines in
spectra of Type Ic supernovae (SNe Ic). The presence of He I lines, with full
width at half maximum ~ 2000 km/s, and the distinct absence of any other
intermediate-width emission (e.g., Halpha), implies that the ejecta of SN Ic
1999cq are interacting with dense circumstellar material composed of almost
pure helium. This strengthens the argument that the progenitors of SNe Ic are
core-collapse events in stars that have lost both their hydrogen and helium
envelopes, either through a dense wind or mass-transfer to a companion. In this
way, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J that
helped firmly establish a physical connection between Type Ib and Type II
supernovae. The light curve of SN 1999cq is very fast, with an extremely rapid
rise followed by a quick decline. SN 1999cq is also found to exhibit a high
level of emission at blue wavelengths (< 5500 A), likely resulting from either
an unusually large amount of iron and iron-group element emission or
uncharacteristically low reddening compared with other SNe Ic.Comment: 17 pages (AASTeX V5.0), 4 figures, accepted for publication in the
Astronomical Journa
Cepheid Calibration of the Peak Brightness of SNe Ia. X. SN 1991T in NGC 4527
Repeated imaging observations have been made of NGC 4527 with the Hubble
Space Telescope between April and June 1999, over an interval of 69 days.
Images were obtained on 12 epochs in the F555W band and on five epochs in the
F814W band. The galaxy hosted the type Ia supernova SN1991T, which showed
relatively unusual behavior by having both an abnormal spectrum near light
maximum, and a slower declining light curve than the proto-typical Branch
normal SNe Ia.
A total of 86 variables that are putative Cepheids have been found, with
periods ranging from 7.4 days to over 70 days. From photometry with the DoPHOT
program, the de-reddened distance modulus is determined to be (m-M)_0 = 30.67
+/- 0.12 (internal uncertainty) using a subset of the Cepheid data whose
reddening and error parameters are secure. A parallel analysis of the Cepheids
using photometry with ROMAFOT yields (m -M)_0 =30.82 +/- 0.11. The final
adopted modulus is (m -M)_0 =30.74 +/- 0.12 +/- 0.12 (d=14.1 +/- 0.8 +/- 0.8
Mpc).
The photometric data for SN1991T are used in combination with the Cepheid
distance to NGC 4527 to obtain the absolute magnitude for this supernova of
M_V^0(max) = -19.85 +/- 0.29. The relatively large uncertainty is a result of
the range in estimates of the reddening to the supernova. Thus SN1991T is seen
to be only moderately brighter (by ~ 0.3 mag) than the mean for
spectroscopically normal supernovae, although magnitude differences of up to
0.6 mag cannot be ruled out.Comment: 46 pages, LATEX using aaspp4.sty, including 9 embedded tables, 19
figures (gif and jpg files), a full-resolution version (ps files) is
available at http://www.astro.unibas.ch/forschung/ll/cepheid.shtml, accepted
for publication in the Astrophysical Journa
Galaxy Orbits for Galaxy Clusters in Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey
We present the results of a study for galaxy orbits in galaxy clusters using
a spectroscopic sample of galaxies in Sloan Digital Sky Survey (SDSS) and 2dF
Galaxy Redshift Survey (2dFGRS). We have determined the member galaxies of
Abell clusters covered by these surveys using the galaxies' redshift and
positional data. We have selected 10 clusters using three criteria: the number
of member galaxies is greater than or equal to 40, the spatial coverage is
complete, and X-ray mass profile is available in the literature. We derive the
radial profile of the galaxy number density and velocity dispersion using all,
early-type, and late-type galaxies for each cluster. We have investigated the
galaxy orbits for our sample clusters with constant and variable velocity
anisotropies over the clustercentric distance using Jeans equation. Using all
member galaxies, the galaxy orbits are found to be isotropic within the
uncertainty for most of sample clusters, although it is difficult to conclude
strongly for some clusters due the large errors and the variation as a function
of the clustercentric distance in the calculated velocity anisotropies. We
investigated the orbital difference between early-type and late-type galaxies
for four sample clusters, and found no significant difference between them.Comment: 59 pages, 21 figures. To appear in ApJ. Paper with high resolution
figures are available at http://astro.kias.re.kr/~hshwang/papers/orbit.pd
Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae
The property of dark energy and the physical reason for acceleration of the
present universe are two of the most difficult problems in modern cosmology.
The dark energy contributes about two-thirds of the critical density of the
present universe from the observations of type-Ia supernova (SNe Ia) and
anisotropy of cosmic microwave background (CMB).The SN Ia observations also
suggest that the universe expanded from a deceleration to an acceleration phase
at some redshift, implying the existence of a nearly uniform component of dark
energy with negative pressure. We use the ``gold'' sample containing 157 SNe Ia
and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the
properties of dark energy and the transition redshift. For a flat universe with
the cosmological constant, we measure , which
is consistent with Riess et al. The transition redshift is
. We also discuss several dark energy models that
define the of the parameterized equation of state of dark energy
including one parameter and two parameters ( being the ratio of the
pressure to energy density). Our calculations show that the accurately
calculated transition redshift varies from to
across these models. We also calculate the minimum
redshift at which the current observations need the universe to
accelerate.Comment: 16 pages, 5 figures, 1 tabl
Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis
We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminosity
stars and substellar objects. Based on these and previously published spectra
we develop a preliminary spectral classification system for L and T dwarfs. For
late L and T types the classification system is based entirely on four spectral
indices in the 1-2.5um interval. Two of these indices are derived from water
absorption bands at 1.15um and 1.4um, the latter of which shows a smooth
increase in depth through the L and T sequences and can be used to classify
both spectral types. The other two indices make use of methane absorption
features in the H and K bands, with the K band index also applicable to mid to
late L dwarfs. Continuum indices shortward of 1um used by previous authors to
classify L dwarfs are found to be useful only through mid L subclasses. We
employ the 1.5um water index and the 2.2um methane index to complete the L
classification through L9.5 and to link the new system with a modified version
of the 2MASS ``Color-d'' index. By correlating the depths of the methane and
water absorption features, we establish a T spectral sequence from types T0 to
T8, based on all four indices, which is a smooth continuation of the L
sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one
SDSS object as L9. In the proposed system methane absorption appears in the K
band approximately at L8, two subclasses earlier than its appearance in the H
band. The L and T spectral classes are distinguished by the absence and
presence, respectively, of H band methane absorption.Comment: 40 pages, 14 figures, to be published in Ap.J., Jan 1, 200
The Origin of the Mass--Metallicity Relation: Insights from 53,000 Star-Forming Galaxies in the SDSS
We utilize Sloan Digital Sky Survey imaging and spectroscopy of ~53,000
star-forming galaxies at z~0.1 to study the relation between stellar mass and
gas-phase metallicity. We derive gas-phase oxygen abundances and stellar masses
using new techniques which make use of the latest stellar evolutionary
synthesis and photoionization models. We find a tight (+/-0.1 dex) correlation
between stellar mass and metallicity spanning over 3 orders of magnitude in
stellar mass and a factor of 10 in metallicity. The relation is relatively
steep from 10^{8.5} - 10^{10.5} M_sun, in good accord with known trends between
luminosity and metallicity, but flattens above 10^{10.5} M_sun. We use indirect
estimates of the gas mass based on the H-alpha luminosity to compare our data
to predictions from simple closed box chemical evolution models. We show that
metal loss is strongly anti-correlated with baryonic mass, with low mass dwarf
galaxies being 5 times more metal-depleted than L* galaxies at z~0.1. Evidence
for metal depletion is not confined to dwarf galaxies, but is found in galaxies
with masses as high as 10^{10} M_sun. We interpret this as strong evidence both
of the ubiquity of galactic winds and of their effectiveness in removing metals
from galaxy potential wells.Comment: ApJ accepted, 15 pages, 9 figures, emulateapj.st
The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data
We report the discovery of a second field methane brown dwarf from the
commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS
J134646.45-003150.4 (SDSS 1346-00), was selected because of its very red color
and stellar appearance. Its spectrum between 0.8-2.5 mic is dominated by strong
absorption bands of H_2O and CH_4 and closely mimics those of Gliese 229B and
SDSS 162414.37+002915.6 (SDSS 1624+00), two other known methane brown dwarfs.
SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that
it lies about 11 pc from the sun. The ratio of flux at 2.1 mic to that at 1.27
mic is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which
suggests that SDSS 1346-00 has a slightly higher effective temperature than the
others. Based on a search area of 130 sq. deg. and a detection limit of z* =
19.8, we estimate a space density of 0.05 pc^-3 for methane brown dwarfs with
T_eff ~ 1000 K in the 40 pc^3 volume of our search. This estimate is based on
small-sample statistics and should be treated with appropriate caution.Comment: 9 pages, 3 figures, AASTeX, to appear in ApJ Letters, authors list
update
Optical Multicolor Photometry of Spectrophotometric Standard Stars
Photoelectric data on the Johnson-Kron-Cousins UBVRI broadband photometric
system are provided for a set of stars which have been used as
spectrophotometric standard stars at the Hubble Space Telescope.Comment: 76 pages, 48 figures - published version available here:
http://www.journals.uchicago.edu/AJ/journal/issues/v133n3/205588/205588.html
. Also see ERRATUM at:
http://www.journals.uchicago.edu/AJ/journal/issues/v133n5/205838/205838.htm
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