268 research outputs found
Superflares on Ordinary Solar-Type Stars
Short duration flares are well known to occur on cool main-sequence stars as
well as on many types of `exotic' stars. Ordinary main-sequence stars are
usually pictured as being static on time scales of millions or billions of
years. Our sun has occasional flares involving up to ergs which
produce optical brightenings too small in amplitude to be detected in
disk-integrated brightness. However, we identify nine cases of superflares
involving to ergs on normal solar-type stars. That is,
these stars are on or near the main-sequence, are of spectral class from F8 to
G8, are single (or in very wide binaries), are not rapid rotators, and are not
exceedingly young in age. This class of stars includes many those recently
discovered to have planets as well as our own Sun, and the consequences for any
life on surrounding planets could be profound. For the case of the Sun,
historical records suggest that no superflares have occurred in the last two
millennia.Comment: 16 pages, accepted for publication in Ap
The Heavy Element Enrichment of Lyman alpha Clouds in the Virgo Supercluster
Using high S/N STIS echelle spectra (FWHM=7 km/s) of 3C 273, we constrain the
metallicities of two Lya clouds in the vicinity of the Virgo cluster. We detect
C II, Si II, and Si III absorption lines in the Lya absorber at z = 0.00530.
Previous observations with FUSE have revealed Ly beta - Ly theta lines at this
redshift, thereby accurately constraining N(H I). We model the ionization of
the gas and derive [C/H] = -1.2^{+0.3}_{-0.2}, [Si/C] = 0.2+/-0.1, and log
n_{H} = -2.8+/-0.3. The model implies a small absorber thickness, ~70 pc, and
thermal pressure p/k ~ 40 cm^{-3} K. It is most likely that the absorber is
pressure confined by an external medium because gravitational confinement would
require a very high ratio of dark matter to baryonic matter. Based on Milky Way
sight lines in which carbon and silicon abundances have been reliably measured
in the same interstellar cloud (including new measurements presented herein),
we argue that the overabundance of Si relative to C is not due to dust
depletion. Instead, this probably indicates that the gas has been predominately
enriched by Type II supernovae. Such enrichment is most plausibly provided by
an unbound galactic wind, given the absence of galaxies within a projected
distance of 100 kpc and the presence of galaxies capable of driving a wind at
larger distances. We also constrain the metallicity and physical conditions of
the Virgo absorber at z = 0.00337 based on detections of O VI and H I and an
upper limit on C IV. If this absorber is collisionally ionized, the O VI/C IV
limit requires T > 10^{5.3} K. For either collisional ionization or
photoionization, we find that [O/H] > -2.0 at z = 0.00337.Comment: Final Ap.J. versio
Evidence of a Supermassive Black Hole in the Galaxy NGC 1023 from the Nuclear Stellar Dynamics
We analyze the nuclear stellar dynamics of the SB0 galaxy NGC 1023, utilizing
observational data both from the Space Telescope Imaging Spectrograph aboard
the Hubble Space Telescope and from the ground. The stellar kinematics measured
from these long-slit spectra show rapid rotation (V = 70 km/s at a distance of
0.1 arcsec = 4.9 pc from the nucleus) and increasing velocity dispersion toward
the nucleus (where sigma = 295 +/- 30 km/s). We model the observed stellar
kinematics assuming an axisymmetric mass distribution with both two and three
integrals of motion. Both modeling techniques point to the presence of a
central dark compact mass (which presumably is a supermassive black hole) with
confidence > 99%. The isotropic two-integral models yield a best-fitting black
hole mass of (6.0 +/- 1.4) x 10^7 M_sun and mass-to-light ratio (M/L_V) of 5.38
+/- 0.08, and the goodness-of-fit (chi^2) is insensitive to reasonable values
for the galaxy's inclination. The three-integral models, which
non-parametrically fit the observed line-of-sight velocity distribution as a
function of position in the galaxy, suggest a black hole mass of (3.9 +/- 0.4)
x 10^7 M_sun and M/L_V of 5.56 +/- 0.02 (internal errors), and the edge-on
models are vastly superior fits over models at other inclinations. The internal
dynamics in NGC 1023 as suggested by our best-fit three-integral model shows
that the velocity distribution function at the nucleus is tangentially
anisotropic, suggesting the presence of a nuclear stellar disk. The nuclear
line of sight velocity distribution has enhanced wings at velocities >= 600
km/s from systemic, suggesting that perhaps we have detected a group of stars
very close to the central dark mass.Comment: 21 pages, 12 figures, accepted in the Astrophysical Journa
UV Absorption Lines from High-Velocity Gas in the Vela Supernova Remnant: New insights from STIS Echelle Observations of HD72089
The star HD72089 is located behind the Vela supernova remnant and shows a
complex array of high and low velocity interstellar absorption features arising
from shocked clouds. A spectrum of this star was recorded over the wavelength
range 1196.4 to 1397.2 Angstroms at a resolving power lambda/Delta lambda =
110,000 and signal-to-noise ratio of 32 by STIS on the Hubble Space Telescope.
We have identified 7 narrow components of C I and have measured their relative
populations in excited fine-structure levels. Broader features at heliocentric
velocities ranging from -70 to +130 km/s are seen in C II, N I, O I, Si II, S
II and Ni II. In the high-velocity components, the unusually low abundances of
N I and O I, relative to S II and Si II, suggest that these elements may be
preferentially ionized to higher stages by radiation from hot gas immediately
behind the shock fronts.Comment: 11 pages, 2 figures, Latex. Submitted for the special HST ERO issue
of the Astrophysical Journal Letter
Observations of H3+ in the Diffuse Interstellar Medium
Surprisingly large column densities of H3+ have been detected using infrared
absorption spectroscopy in seven diffuse cloud sightlines (Cygnus OB2 12,
Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating
that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard
model of diffuse cloud chemistry, our H3+ column densities imply unreasonably
long path lengths (~1 kpc) and low densities (~3 cm^-3). Complimentary
millimeter-wave, infrared, and visible observations of related species suggest
that the chemical model is incorrect and that the number density of H3+ must be
increased by one to two orders of magnitude. Possible solutions include a
reduced electron fraction, an enhanced rate of H2 ionization, and/or a smaller
value of the H3+ dissociative recombination rate constant than implied by
laboratory experiments.Comment: To be published in Astrophysical Journal, March 200
Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
Ultraviolet absorption from interstellar 12CO and 13CO was detected toward
rho Oph A and chi Oph. The measurements were obtained at medium resolution with
the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column
density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for
the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm
600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained.
Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this
star as well.
The ratios are larger than the isotopic ratios for carbon and oxygen
appropriate for ambient interstellar material. Since for both carbon and oxygen
the more abundant isotopomer is enhanced, selective isotopic photodissociation
plays the key role in the fractionation process for these directions. The
enhancement arises because the more abundant isotopomer has lines that are more
optically thick, resulting in more self shielding from dissociating radiation.
A simple argument involving the amount of self shielding [from N(12CO)] and the
strength of the ultraviolet radiation field premeating the gas (from the amount
of vibrationally excited H_2) shows that selective isotopic photodissociation
controls the fractionation seen in these two sight lines, as well as the sight
line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap
A Pair of Compact Red Galaxies at Redshift 2.38, Immersed in a 100 kpc Scale Ly-alpha Nebula
We present Hubble Space Telescope (HST) and ground-based observations of a
pair of galaxies at redshift 2.38, which are collectively known as 2142-4420 B1
(Francis et al. 1996). The two galaxies are both luminous extremely red objects
(EROs), separated by 0.8 arcsec. They are embedded within a 100 kpc scale
diffuse Ly-alpha nebula (or blob) of luminosity ~10^44 erg/s.
The radial profiles and colors of both red objects are most naturally
explained if they are young elliptical galaxies: the most distant yet found. It
is not, however, possible to rule out a model in which they are abnormally
compact, extremely dusty starbursting disk galaxies. If they are elliptical
galaxies, their stellar populations have inferred masses of ~10^11 solar masses
and ages of ~7x10^8 years. Both galaxies have color gradients: their centers
are significantly bluer than their outer regions. The surface brightness of
both galaxies is roughly an order of magnitude greater than would be predicted
by the Kormendy relation. A chain of diffuse star formation extending 1 arcsec
from the galaxies may be evidence that they are interacting or merging.
The Ly-alpha nebula surrounding the galaxies shows apparent velocity
substructure of amplitude ~ 700 km/s. We propose that the Ly-alpha emission
from this nebula may be produced by fast shocks, powered either by a galactic
superwind or by the release of gravitational potential energy.Comment: 33 pages, 9 figures, ApJ in press (to appear in Jun 10 issue
Spatially Resolved STIS Spectroscopy of SN 1987A: Evidence for Shock Interaction with Circumstellar Gas
Visual and ultraviolet spatially resolved (~ 0."1) spectra of SN 1987A
obtained on days 3715 and 3743 with the Space Telescope Imaging Spectrograph on
the Hubble Space Telescope show that the high-velocity SN debris is colliding
with circumstellar gas. Very broad Ly-alpha emission with velocities extending
to ~ +/- 20,000 km/s originates inside the inner circumstellar ring and appears
to fill most of the surface area within 0."67 +/- 0."03 (0.14 pc at a distance
of 50 kpc) of the ring's center. The observed Ly-alpha flux from the shocked
ejecta is (1.85 +/- 0.53) 10^{-13} erg/cm2/s and (1.25 +/- 0.51) 10^{-12}
erg/cm2/s after correcting for extinction. A spatially unresolved blue-shifted
emission feature was discovered in H-alpha (and other lines) on the inner ring
at p.a. 31 +/- 8 degree. The H-alpha emission extends to -250 km/s with no
corresponding red-shifted emission. This highly localized interaction appears
to be the initial contact of the supernova blast wave with an inward protrusion
of the inner ring. The broad Ly-alpha emission and the `hot spot' are separate
interaction phemonena associated with the reverse and forward shocks,
respectively. We also find that the size of the inner ring in forbidden lines
of oxygen has a dependence on ionization potential, in agreement with
photoionization models of the ring.Comment: 11 pages (LaTeX, aaspp4.sty), 8 figures, accepted for publication in
the Astrophysical Journal Letters HST Second Servicing Mission special issu
Complex C: A Low-Metallicity High-Velocity Cloud Plunging into the Milky Way
(Abridged) We present a new high-resolution (7 km/s FWHM) echelle spectrum of
3C 351 obtained with STIS. 3C 351 lies behind the low-latitude edge of
high-velocity cloud Complex C, and the new spectrum provides accurate
measurements of O I, Si II, Al II, Fe II, and Si III absorption lines at the
velocity of the HVC. We use collisional and photoionization models to derive
ionization corrections; in both models we find that the overall metallicity Z =
0.1 - 0.3 Z_{solar} in Complex C, but nitrogen must be underabundant. The iron
abundance indicates that Complex C contains very little dust. The absorbing gas
probably is not gravitationally confined. The gas could be pressure-confined by
an external medium, but alternatively we may be viewing the leading edge of the
HVC, which is ablating and dissipating as it plunges into the Milky Way. O VI
column densities observed with FUSE toward nine QSOs/AGNs behind Complex C
support this conclusion: N(O VI) is highest near 3C 351, and the O VI/H I ratio
increases substantially with decreasing latitude, suggesting that the
lower-latitude portion of the cloud is interacting more vigorously with the
Galaxy. The other sight lines through Complex C show some dispersion in
metallicity, but with the current uncertainties, the measurements are
consistent with a constant metallicity throughout the HVC. However, all of the
Complex C sight lines require significant nitrogen underabundances. Finally, we
compare the 3C 351 sight line to the sight line to the nearby QSO H1821+643 to
search for evidence of outflowing Galactic fountain gas that could be mixing
with Complex C. We find that the intermediate-velocity gas detected toward 3C
351 and H1821+643 has a higher metallicity and may well be a fountain/chimney
outflow from the Perseus spiral arm.Comment: Submitted to AJ. Figures 1-4 compressed for astro-ph; better quality
figures are available at
http://astro.princeton.edu/~tripp/astro/qualitypreps/complexc.ps.g
The Nuclear Dynamics of M32. I. Data and Stellar Kinematics
We have obtained optical long-slit spectroscopy of the nucleus of M32 using
the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. The
stellar rotation velocity and velocity dispersion, as well as the full
line-of-sight velocity distribution (LOSVD), were determined as a function of
position along the slit using two independent spectral deconvolution
algorithms. We see three clear kinematical signatures of the nuclear black
hole: a sudden upturn, at ~0.3 arc seconds from the center, in the stellar
velocity dispersions; a flat or rising rotation curve into the center; and
strong, non-Gaussian wings on the central LOSVD. The central velocity
dispersion is ~130 km/s (Gaussian fit) or ~175 km/s (corrected for the wings).
Both the velocity dispersion spike and the shape of the central LOSVD are
consistent with the presence of a supermassive compact object in M32 with a
mass in the range 2-5 x 10^6 solar masses. These data are a significant
improvement on previous stellar kinematical data, making M32 the first galaxy
for which the imprint of the black hole's gravitation on the stellar velocities
has been observed with a resolution comparable to that of gas-dynamical
studies.Comment: 55 pages, 17 figures. Submitted to The Astrophysical Journa
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