160 research outputs found
The Far-Ultraviolet Spectra of TW Hya. II. Models of H2 Fluorescence in a Disk
We measure the temperature of warm gas at planet-forming radii in the disk
around the classical T Tauri star (CTTS) TW Hya by modelling the H2
fluorescence observed in HST/STIS and FUSE spectra. Strong Ly-alpha emission
irradiates a warm disk surface within 2 AU of the central star and pumps
certain excited levels of H2. We simulate a 1D plane-parallel atmosphere to
estimate fluxes for the 140 observed H2 emission lines and to reconstruct the
Ly-alpha emission profile incident upon the warm H2. The excitation of H2 can
be determined from relative line strengths by measuring self-absorption in
lines with low-energy lower levels, or by reconstructing the Ly-alpha profile
incident upon the warm H2 using the total flux from a single upper level and
the opacity in the pumping transition. Based on those diagnostics, we estimate
that the warm disk surface has a column density of log
N(H2)=18.5^{+1.2}_{-0.8}, a temperature T=2500^{+700}_{-500} K, and a filling
factor of H2, as seen by the source of Ly-alpha emission, of 0.25\pm0.08 (all
2-sigma error bars). TW Hya produces approximately 10^{-3} L_\odot in the FUV,
about 85% of which is in the Ly-alpha emission line. From the H I absorption
observed in the Ly-alpha emission, we infer that dust extinction in our line of
sight to TW Hya is negligible.Comment: Accepted by ApJ. 26 pages, 17 figures, 6 table
The FUV spectrum of TW Hya. I. Observations of H Fluorescence
We observed the classical T Tauri star TW Hya with \textit{HST}/STIS using
the E140M grating, from 1150--1700 \AA, with the E230M grating, from 2200--2900
\AA, and with \FUSE from 900--1180 \AA. Emission in 143 Lyman-band H lines
representing 19 progressions dominates the spectral region from 1250--1650 \AA.
The total H emission line flux is erg cm
s, which corresponds to at TW Hya's
distance of 56 pc. A broad stellar \Lya line photoexcites the H from
excited rovibrational levels of the ground electronic state to excited
electronic states. The \ion{C}{2} 1335 \AA doublet, \ion{C}{3} 1175 \AA\
multiplet, and \ion{C}{4} 1550 \AA doublet also electronically excite H.
The velocity shift of the H lines is consistent with the photospheric
radial velocity of TW Hya, and the emission is not spatially extended beyond
the 0\farcs05 resolution of \textit{HST}. The H lines have an intrinsic
FWHM of \kms. One H line is significantly weaker than
predicted by this model because of \ion{C}{2} wind absorption. We also do not
observe any H absorption against the stellar \Lya profile. From these
results, we conclude that the H emission is more consistent with an origin
in a disk rather than in an outflow or circumstellar shell. We also analyze the
hot accretion-region lines (e.g., \ion{C}{4}, \ion{Si}{4}, \ion{O}{6}) of TW
Hya, which are formed at the accretion shock, and discuss some reasons why Si
lines appear significantly weaker than other TR region lines.Comment: accepted by ApJ, 42 pages -- 20 text, 11 figure
Metal Depletion and Warm H2 in the Brown Dwarf 2M1207 Accretion Disk
We present new far-ultraviolet observations of the young M8 brown dwarf 2MASS
J12073346-3932539, which is surrounded by an accretion disk. The data were
obtained using the Hubble Space Telescope-Cosmic Origins Spectrograph. Moderate
resolution spectra (R~17,000-18,000) obtained in the 1150-1750 A and 2770-2830
A bandpasses reveal H2 emission excited by HI Ly photons, several
ionization states of carbon (CI - CIV), and hot gas emission lines of HeII and
NV (T ~ 10^4-5 K). Emission from some species that would be found in a typical
thermal plasma at this temperature (SiII, SiIII, SiIV, and MgII) are not
detected. The non-detections indicate that these refractory elements are
depleted into grains, and that accretion shocks dominate the production of the
hot gas observed on 2MASS J12073346-3932539. We use the observed CIV luminosity
to constrain the mass accretion rate in this system. We use the kinematically
broadened H2 profile to confirm that the majority of the molecular emission
arises in the disk, measure the radius of the inner hole of the disk
(R_{hole}~3R_{*}), and constrain the physical conditions of the warm molecular
phase of the disk (T(H2)~2500-4000 K). A second, most likely unresolved H2
component is identified. This feature is either near the stellar surface in the
region of the accretion shock or in a molecular outflow, although the
possibility that this Jovian-like emission arises on the day-side disk of a 6
M_{J} companion (2M1207b) cannot be conclusively ruled out. In general, we find
that this young brown dwarf disk system is a low-mass analog to classical T
Tauri stars that are observed to produce H2 emission from a warm layer in their
disks, such as the well studied TW Hya and DF Tau systems.Comment: ApJ, accepted. 12 pages, 10 figures. 3 tables
The Active Corona of HD 35850 (F8 V)
We present Extreme Ultraviolet Explorer spectroscopy and photometry of the
nearby F8 V star HD 35850 (HR 1817). The EUVE spectra reveal 28 emission lines
from Fe IX and Fe XV to Fe XXIV. The Fe XXI 102, 129 A ratio yields an upper
limit for the coronal electron density, log n < 11.6 per cc. The EUVE SW
spectrum shows a small but clearly detectable continuum. The line-to-continuum
ratio indicates approximately solar Fe abundances, 0.8 < Z < 1.6. The resulting
emission-measure distribution is characterized by two temperature components at
log T of 6.8 and 7.4. The EUVE spectra have been compared with non-simultaneous
ASCA SIS spectra of HD 35850. The SIS spectrum shows the same temperature
distribution as the EUVE DEM analysis. However, the SIS spectral firs suggest
sub-solar abundances, 0.34 < Z < 0.81. Although some of the discrepancy may be
the result of incomplete X-ray line lists, we cannot explain the disagreement
between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance.
Given its youth (t ~ 100 Myr), its rapid rotation (v sin i ~ 50 km/s), and its
high X-ray activity (Lx ~ 1.5E+30 ergs/s), HD 35850 may represent an activity
extremum for single, main-sequence F-type stars. The variability and EM
distribution can be reconstructed using the continuous flaring model of Guedel
provided that the flare distribution has a power-law index of 1.8. Similar
results obtained for other young solar analogs suggest that continuous flaring
is a viable coronal heating mechanism on rapidly rotating, late-type,
main-sequence stars.Comment: 32 pages incl. 14 figures and 3 tables. To appear in the 1999 April
10 issue of The Astrophysical Journa
The Very Young Type Ia Supernova 2013dy: Discovery, and Strong Carbon Absorption in Early-Time Spectra
The Type Ia supernova (SN Ia) 2013dy in NGC 7250 (d ~ 13.7 Mpc) was
discovered by the Lick Observatory Supernova Search. Combined with a
prediscovery detection by the Italian Supernova Search Project, we are able to
constrain the first-light time of SN 2013dy to be only 0.10 +/- 0.05 d (2.4 +/-
1.2 hr) before the first detection. This makes SN 2013dy the earliest known
detection of an SN Ia. We infer an upper limit on the radius of the progenitor
star of R_0 < 0.25 R_sun, consistent with that of a white dwarf. The light
curve exhibits a broken power law with exponents of 0.88 and then 1.80. A
spectrum taken 1.63 d after first light reveals a C II absorption line
comparable in strength to Si II. This is the strongest C II feature ever
detected in a normal SN Ia, suggesting that the progenitor star had significant
unburned material. The C II line in SN 2013dy weakens rapidly and is undetected
in a spectrum 7 days later, indicating that C II is detectable for only a very
short time in some SNe Ia. SN 2013dy reached a B-band maximum of M_B = -18.72
+/- 0.03 mag ~17.7 d after first light.Comment: Accepted for Publication in ApJ Letter
Recommended from our members
Early Observations And Analysis Of The Type Ia SN 2014J In M82
We present optical and near infrared (NIR) observations of the nearby Type Ia SN 2014J. Seventeen optical and 23 NIR spectra were obtained from 10 days before (-10d) to 10 days after (+10d) the time of maximum B-band brightness. The relative strengths of absorption features and their patterns of development can be compared at one day intervals throughout most of this period. Carbon is not detected in the optical spectra, but we identify C I lambda 1.0693 in the NIR spectra. Mg II lines with high oscillator strengths have higher initial velocities than other Mg II lines. We show that the velocity differences can be explained by differences in optical depths due to oscillator strengths. The spectra of SN 2014J show that it is a normal SN Ia, but many parameters are near the boundaries between normal and high-velocity subclasses. The velocities for OI, Mg II, Si II, S Ca a, and Fell suggest that SN 2014J has a layered structure with little or no mixing. That result is consistent with the delayed detonation explosion models. We also report photometric observations, obtained from -10d to +29d, in the UBVRIJH and K-s bands. The template fitting package SNooPy is used to interpret the light curves and to derive photometric parameters. Using R-v = 1.46, which is consistent with previous studies, SNooPy finds that A(v) = 1.80 for E(B - V)(host) = 1.23 +/- 0.06 mag. The maximum B-band brightness of -19.19 +/- 0.10 mag was reached on February 1.74 UT +/- 0.13 days and the supernova has a decline parameter, Delta m(15), of 1.12 +/- 0.02 mag.Department of Space, Government of IndiaHungarian OTKA NN-107637NSF AST-1109801, AST-1151462, AST-1211196NSF Astronomy and Astrophysics Postdoctoral Fellowship AST-1302771NASA through a grant from the Space Telescope Science Institute GO-12540NASA NAS5-26555Swedish Research CouncilSwedish National Space BoardDanish Agency for Science and Technology and Innovation realized through a Sapere Aude Level 2 grantAstronom
WIYN Open Cluster Study. XXXIX. Abundances in NGC 6253 from HYDRA Spectroscopy of the Li 6708 A Region
High-dispersion spectra of 89 potential members of the old, super-metal-rich
open cluster, NGC 6253, have been obtained with the HYDRA multi-object
spectrograph. Based upon radial-velocity measurements alone, 47 stars at the
turnoff of the cluster color-magnitude diagram (CMD) and 18 giants are
identified as potential members. Five turnoff stars exhibit evidence of
binarity while proper-motion data eliminates two of the dwarfs as members. The
mean cluster radial velocity from probable single-star members is -29.4 +/- 1.3
km/sec (sd). A discussion of the current estimates for the cluster reddening,
derived independently of potential issues with the BV cluster photometry, lead
to an adopted reddening of E(B-V) = 0.22 +/- 0.04. From equivalent width
analyses of 38 probable single-star members near the CMD turnoff, the weighted
average abundances are found to be [Fe/H] = +0.43 +/- 0.01, [Ni/H] = +0.53 +/-
0.02 and [Si/H] = +0.43 (+0.03,-0.04), where the errors refer to the standard
errors of the weighted mean. Weak evidence is found for a possible decline in
metallicity with increasing luminosity among stars at the turnoff. We discuss
the possibility that our turnoff stars have been affected by microscopic
diffusion. For 15 probable single-star members among the giants, spectrum
synthesis leads to abundances of +0.46 (+0.02,-0.03) for [Fe/H]. While less
than half the age of NGC 6791, NGC 6253 is at least as metal-rich and, within
the uncertainties, exhibits the same general abundance pattern as that typified
by super-metal-rich dwarfs of the galactic bulge.Comment: 5 Tables, 9 figures, 45 page
Carbon Detection in Early-Time Optical Spectra of Type Ia Supernovae
While O is often seen in spectra of Type Ia supernovae (SNe Ia) as both
unburned fuel and a product of C burning, C is only occasionally seen at the
earliest times, and it represents the most direct way of investigating
primordial white dwarf material and its relation to SN Ia explosion scenarios
and mechanisms. In this paper, we search for C absorption features in 188
optical spectra of 144 low-redshift (z < 0.1) SNe Ia with ages <3.6 d after
maximum brightness. These data were obtained as part of the Berkeley SN Ia
Program (BSNIP; Silverman et al. 2012) and represent the largest set of SNe Ia
in which C has ever been searched. We find that ~11 per cent of the SNe studied
show definite C absorption features while ~25 per cent show some evidence for C
II in their spectra. Also, if one obtains a spectrum at t < -5 d, then there is
a better than 30 per cent chance of detecting a distinct absorption feature
from C II. SNe Ia that show C are found to resemble those without C in many
respects, but objects with C tend to have bluer optical colours than those
without C. The typical expansion velocity of the C II {\lambda}6580 feature is
measured to be 12,000-13,000 km/s, and the ratio of the C II {\lambda}6580 to
Si II {\lambda}6355 velocities is remarkably constant with time and among
different objects with a median value of ~1.05. While the pseudo-equivalent
widths (pEWs) of the C II {\lambda}6580 and C II {\lambda}7234 features are
found mostly to decrease with time, we see evidence of a significant increase
in pEW between ~12 and 11 d before maximum brightness, which is actually
predicted by some theoretical models. The range of pEWs measured from the BSNIP
data implies a range of C mass in SN Ia ejecta of about (2-30) * 10^-3 M_Sun.Comment: 20 pages, 11 figures, 4 tables, revised version re-submitted to MNRA
Simultaneous Extreme-Ultraviolet Explorer and Optical Observations of Ad Leonis: Evidence for Large Coronal Loops and the Neupert Effect in Stellar Flares
We report on the first simultaneous Extreme-Ultraviolet Explorer (EUVE) and optical observations of flares on the dMe flare star AD Leonis. The data show the following features: (1) Two flares (one large and one of moderate size) of several hours duration were observed in the EUV wavelength range; (2) Flare emission observed in the optical precedes the emission seen with EUVE; and (3) Several diminutions (DIMs) in the optical continuum were observed during the period of optical flare activity. To interpret these data, we develop a technique for deriving the coronal loop length from the observed rise and decay behavior of the EUV flare. The technique is generally applicable to existing and future coronal observations of stellar flares. We also determine the pressure, column depth, emission measure, loop cross-sectional area, and peak thermal energy during the two EUV flares, and the temperature, area coverage, and energy of the optical continuum emission. When the optical and coronal data are combined, we find convincing evidence of a stellar 'Neupert effect' which is a strong signature of chromospheric evaporation models. We then argue that the known spatial correlation of white-light emission with hard X-ray emission in solar flares, and the identification of the hard X-ray emission with nonthermal bremsstrahlung produced by accelerated electrons, provides evidence that flare heating on dMe stars is produced by the same electron precipitation mechanism that is inferred to occur on the Sun. We provide a thorough picture of the physical processes that are operative during the largest EUV flare, compare and contrast this picture with the canonical solar flare model, and conclude that the coronal loop length may be the most important factor in determining the flare rise time and energetics
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