521 research outputs found
Discovery of Radio Outbursts in the Active Nucleus of M81
The low-luminosity active galactic nucleus of M81 has been monitored at
centimeter wavelengths since early 1993 as a by-product of radio programs to
study the radio emission from Supernova 1993J. The extensive data sets reveal
that the nucleus experienced several radio outbursts during the monitoring
period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of
1993 September and lasted for approximately three months; at longer
wavelengths, the maximum flux density decreases, and the onset of the burst is
delayed. These characteristics qualitatively resemble the standard model for
adiabatically expanding radio sources, although certain discrepancies between
the observations and the theoretical predictions suggest that the model is too
simplistic. In addition to the large-amplitude, prolonged variations, we also
detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on
short (less than 1 day) timescales. We discuss a possible association between
the radio activity and an optical flare observed during the period that the
nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including
embedded figures and table
X-ray, Optical, and Radio Observations of the Type II Supernovae 1999em and 1998S
Observations of the Type II-P (plateau) Supernova (SN) 1999em and Type IIn
(narrow emission line) SN 1998S have enabled estimation of the profile of the
SN ejecta, the structure of the circumstellar medium (CSM) established by the
pre-SN stellar wind, and the nature of the shock interaction. SN 1999em is the
first Type II-P detected at both X-ray and radio wavelengths. The Chandra X-ray
data indicate non-radiative interaction of SN ejecta with a power-law density
profile (rho \propto r^{-n} with n ~ 7) with a pre-SN wind with a low mass-loss
rate of ~2 \times 10^{-6} M_sun/yr for a wind velocity of 10 km/sec, in
agreement with radio mass-loss rate estimates. The Chandra data show an
unexpected, temporary rise in the 0.4--2.0 keV X-ray flux at ~100 days after
explosion. SN 1998S, at an age of >3 years, is still bright in X-rays and is
increasing in flux density at cm radio wavelengths. Spectral fits to the
Chandra data show that many heavy elements (Ne, Al, Si, S, Ar, and Fe) are
overabundant with respect to solar values. We compare the observed elemental
abundances and abundance ratios to theoretical calculations and find that our
data are consistent with a progenitor mass of approximately 15-20 M_sun if the
heavy element ejecta are radially mixed out to a high velocity. If the X-ray
emission is from the reverse shock wave region, the supernova density profile
must be moderately flat at a velocity ~10^4 km/sec, the shock front is
non-radiative at the time of the observations, and the mass-loss rate is 1-2
\times 10^{-4} M_sun/yr for a pre-supernova wind velocity of 10 km/sec. This
result is also supported by modeling of the radio emission which implies that
SN 1998S is surrounded by a clumpy or filamentary CSM established by a high
mass-loss rate, ~2 \times 10^{-4} M_sun/yr, from the pre-supernova star.Comment: 14 pages, 10 figures, accepted by ApJ, includes new data (one new
obs. each of SN 1999em and SN 1998S), expanded discussion of spectral fit
The Environments of Supernovae in Post-Refurbishment Hubble Space Telescope Images
The locations of supernovae in the local stellar and gaseous environment in
galaxies contain important clues to their progenitor stars. Access to this
information, however, has been hampered by the limited resolution achieved by
ground-based observations. High spatial resolution Hubble Space Telescope (HST)
images of galaxy fields in which supernovae had been observed can improve the
situation considerably. We have examined the immediate environments of a few
dozen supernovae using archival post-refurbishment HST images. Although our
analysis is limited due to signal-to-noise ratio and filter bandpass
considerations, the images allow us for the first time to resolve individual
stars in, and to derive detailed color-magnitude diagrams for, several
environments. We are able to place more rigorous constraints on the masses of
these supernovae. A search was made for late-time emission from supernovae in
the archival images, and for the progenitor stars in presupernova images of the
host galaxies. We have detected SN 1986J in NGC 891 and, possibly, SN 1981K in
NGC 4258. We have also identified the progenitor of the Type IIn SN 1997bs in
NGC 3627. By removing younger resolved stars in the environments of SNe Ia, we
can measure the colors of the unresolved stellar background and attribute these
colors generally to an older, redder population. HST images ``accidentally''
caught the Type Ia SN 1994D in NGC 4526 shortly after its outburst; we measure
its brightness. Finally, we add to the statistical inferences that can be made
from studying the association of SNe with recent star-forming regions.Comment: 20 pages, 29 figures, to appear in A
Type II Supernovae as Probes of Cosmology
- Constraining the cosmological parameters and understanding Dark Energy have
tremendous implications for the nature of the Universe and its physical laws.
- The pervasive limit of systematic uncertainties reached by cosmography
based on Cepheids and Type Ia supernovae (SNe Ia) warrants a search for
complementary approaches.
- Type II SNe have been shown to offer such a path. Their distances can be
well constrained by luminosity-based or geometric methods. Competing,
complementary, and concerted efforts are underway, to explore and exploit those
objects that are extremely well matched to next generation facilities.
Spectroscopic follow-up will be enabled by space- based and 20-40 meter class
telescopes.
- Some systematic uncertainties of Type II SNe, such as reddening by dust and
metallicity effects, are bound to be different from those of SNe Ia. Their
stellar progenitors are known, promising better leverage on cosmic evolution.
In addition, their rate - which closely tracks the ongoing star formation rate
- is expected to rise significantly with look- back time, ensuring an adequate
supply of distant examples.
- These data will competitively constrain the dark energy equation of state,
allow the determination of the Hubble constant to 5%, and promote our
understanding of the processes involved in the last dramatic phases of massive
stellar evolution.Comment: Science white paper, submitted to the Decadal committee Astro201
Supernova 1954J (Variable 12) in NGC 2403 Unmasked
We have confirmed that the precursor star of the unusual Supernova 1954J
(also known as Variable 12) in NGC 2403 survived what appears to have been a
super-outburst, similar to the 1843 Great Eruption of eta Carinae in the
Galaxy. The apparent survivor has changed little in brightness and color over
the last eight years, and a Keck spectrum reveals characteristics broadly
similar to those of eta Car. This is further suggested by our identification of
the actual outburst-surviving star in high-resolution images obtained with the
Advanced Camera for Surveys on the Hubble Space Telescope. We reveal this
``supernova impostor'' as a highly luminous (M_V^0 ~ -8.0 mag), very massive
(M_initial >~ 25 Msun) eruptive star, now surrounded by a dusty (A_V ~ 4 mag)
nebula, similar to eta Car's famous Homunculus.Comment: 13 pages, 10 figures, to appear in the 2005 June PAS
What Powers the 3000-Day Light Curve of SN 2006gy?
SN 2006gy was the most luminous supernova (SN) ever observed at the time of its discovery and the first of the newly defined class of superluminous supernovae (SLSNe). The extraordinary energetics of SN 2006gy and all SLSNe (>10^(51) erg) require either atypically large explosion energies (e.g. pair-instability explosion) or the efficient conversion of kinetic into radiative energy (e.g. shock interaction). The mass-loss characteristics can therefore offer important clues regarding the progenitor system. For the case of SN 2006gy, both a scattered and thermal light echo from circumstellar material (CSM) have been reported at later epochs (day ∼800), ruling out the likelihood of a pair-instability event and leading to constraints on the characteristics of the CSM. Owing to the proximity of the SN to the bright host-galaxy nucleus, continued monitoring of the light echo has not been trivial, requiring the high resolution offered by the Hubble Space Telescope (HST) or ground-based adaptive optics (AO). Here, we report detections of SN 2006gy using HST and Keck AO at ∼3000 d post-explosion and consider the emission mechanism for the very late-time light curve. While the optical light curve and optical spectral energy distribution are consistent with a continued scattered-light echo, a thermal echo is insufficient to power the K′-band emission by day 3000. Instead, we present evidence for late-time infrared emission from dust that is radiatively heated by CSM interaction within an extremely dense dust shell, and we consider the implications on the CSM characteristics and progenitor system
First Views of a Nearby LIRG: Star Formation and Molecular Gas in IRAS 04296+2923
We present a first look at the local LIRG, IRAS04296+2923. This barred
spiral, overlooked because of its location in the Galactic plane, is among the
half dozen closest LIRGs. More IR-luminous than either M82 or the Antennae, it
may be the best local example of a nuclear starburst caused by bar-mediated
secular evolution. We present Palomar J and Pa beta images, VLA maps from
20-1.3cm, a Keck LWS image at 11.7mic and OVRO CO(1-0) and ^13CO(1-0), and 2.7
mm continuum images. The J-band image shows a symmetric barred spiral. Two
bright, compact mid-IR/radio sources in the nucleus comprise a starburst that
is equivalent to 10^5 O7 stars, probably a pair of young super star clusters
separated by 30pc. The nuclear starburst is forming stars at the rate of
~12Msun/yr, half of the total star formation rate for the galaxy of ~25Msun/yr.
IRAS04296 is bright in CO, and among the most gas-rich galaxies in the local
universe. The CO luminosity of the inner half kpc is equivalent to that of the
entire Milky Way. While the most intense CO emission extends over a 15"(2 kpc)
region, the nuclear starburst is confined to ~1-2"(150-250 pc) of the dynamical
center. From ^13CO, we find that the CO conversion factor in the nucleus is
higher than the Galactic value by a factor 3-4, typical of gas-rich spiral
nuclei. The nuclear star formation efficiency is M_gas/SFR^nuc = 2.7x10^-8
yr^-1, corresponding to gas consumption timescale, tau_SF^nuc~4x10^7 yrs. The
star formation efficiency is ten times lower in the disk, tau_SF^disk~3.3x10^8
yrs. The low absolute star formation efficiency in the disk implies that the
molecular gas is not completely consumed before it drifts into the nucleus, and
is capable of fueling a sustained nuclear starburst. IRAS04296 is beginning a
100Myr period as a LIRG, during which it will turn much of its 6x10^9Msun of
molecular gas into a nuclear cluster of stars. (abridged)Comment: Accepted, Astronomical Journa
Daily Stress Processes as Potential Intervention Targets to Reduce Gender Differences and Improve Mental Health Outcomes in Mid- and Later Life.
The current study examines daily stress processes as risk factors for comprised mental health in midlife and later life, specifically for gender differences in depression risk. Using data from the Midlife in the United States (MIDUS) study and the National Study of Daily Experiences (NSDE), we examine (1) gender differences in depression; (2) the prospective effects of differential exposure and affective responses on 10-year depression status; (3) gender differences in daily stress-depression links. Furthermore, we explore whether the protective factor of help-seeking behavior moderates the effects of daily stress on depression. Participants included 1289 (mage = 55; SD = 12; range = 34-83; 56% female) individuals who completed the second waves of MIDUS and the 8-day NSDE daily diary protocol and participated in the third wave of MIDUS approximately 10 years later. Respondents completed assessments of depression and their seeking assistance from a psychiatrist, mental health professional, counselor, or religious leader. Covariate-adjusted logistic regression analyses revealed increased odds of depression among women compared to men, but no significant gender difference after taking daily stress into account. Higher levels of stressor exposure, negative affect, and affective reactivity were associated with increased odds of depression for both men and women. Compared to those who did not engage in help-seeking behavior, those who did had significantly greater odds of depression, and there were asymmetric patterns of daily stress effects across groups. These findings highlight differential exposure, negative affect, and affective responses to daily stress as potentially accessible intervention targets for reducing stress in daily life and mitigating longer-term depression risk during mid- and later life
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