20 research outputs found
A White Light Megaflare on the dM4.5e Star YZ CMi
On UT 2009 January 16, we observed a white light megaflare on the dM4.5e star
YZ CMi as part of a long-term spectroscopic flare-monitoring campaign to
constrain the spectral shape of optical flare continuum emission. Simultaneous
U-band photometric and 3350A-9260A spectroscopic observations were obtained
during 1.3 hours of the flare decay. The event persisted for more than 7 hours
and at flare peak, the U-band flux was almost 6 magnitudes brighter than in the
quiescent state. The properties of this flare mark it as one of the most
energetic and longest-lasting white light flares ever to be observed on an
isolated low-mass star. We present the U-band flare energetics and a flare
continuum analysis. For the first time, we show convincingly with spectra that
the shape of the blue continuum from 3350A to 4800A can be represented as a sum
of two components: a Balmer continuum as predicted by the Allred et al
radiative hydrodynamic flare models and a T ~ 10,000K blackbody emission
component as suggested by many previous studies of the broadband colors and
spectral distributions of flares. The areal coverage of the Balmer continuum
and blackbody emission regions vary during the flare decay, with the Balmer
continuum emitting region always being significantly (~3-16 times) larger.
These data will provide critical constraints for understanding the physics
underlying the mysterious blue continuum radiation in stellar flares.Comment: 12 pages, 4 figures, accepted by Astrophysical Journal Letter
Observations and modelling of a large optical flare on AT Microscopii
Spectroscopic observations covering the wavelength range 3600--4600\AA are
presented for a large flare on the late type M dwarf AT Mic (dM4.5e). A
procedure to estimate the physical parameters of the flaring plasma has been
used which assumes a simplified slab model of the flare based on a comparison
of observed and computed Balmer decrements. With this procedure we have
determined the electron density, electron temperature, optical thickness and
temperature of the underlying source for the impulsive and gradual phases of
the flare. The magnitude and duration of the flare allows us to trace the
physical parameters of the response of the lower atmosphere. In order to check
our derived values we have compared them with other methods. In addition, we
have also applied our procedure to a stellar and a solar flare for which
parameters have been obtained using other techniques.Comment: 11 pages, 8 tables, accepted by A&
Near-Ultraviolet Spectra of Flares on YZ CMi
Near-ultraviolet spectroscopic data obtained with the HST STIS instrument on
the dMe flare star YZ Canis Minoris (YZ CMi) were analyzed. Flare and quiet
intervals were identified from the broadband near-UV light curve, and the
spectrum of each flare was separately extracted. Mg II and Fe II line profiles
show similar behavior during the flares. Two large flares allowed time-resolved
spectra to be analyzed, revealing a very broad component to the Mg II k line
profile in at least one flare spectrum (F9b). If interpreted as a velocity,
this component requires chromospheric material to be moving with FWHM ~ 250
km/sec, implying kinetic energy far in excess of the radiative energy. The Mg
II k flare line profiles were compared to recent radiative hydrodynamic models
of flare atmospheres undergoing electron beam heating. The models successfully
predict red enhancements in the line profile with typical velocity of a few
km/sec, but do not reproduce the flares showing blue enhancements, or the
strongly broadened line observed in flare F9b. A more complete calculation of
redistribution into the line wings, including the effect of collisions with the
electron beam, may resolve the origin of the excess line broadening.Comment: 29 pages, 12 figures; accepted to PAS
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
Analysis and modeling of high temporal resolution spectroscopic observations of flares on AD Leo
We report the results of a high temporal resolution spectroscopic monitoring
of the flare star AD Leo. During 4 nights, more than 600 spectra were taken in
the optical range using the Isaac Newton Telescope (INT) and the Intermediate
Dispersion Spectrograph (IDS). We have observed a large number of short and
weak flares occurring very frequently (flare activity > 0.71 hours-1). This is
in favour of the very important role that flares can play in stellar coronal
heating. The detected flares are non white-light flares and, though most of
solar flares belong to this kind, very few such events had been previously
observed on stars. The behaviour of different chromospheric lines (Balmer
series from H_alpha to H_11, Ca II H & K, Na I D_1 & D_2, He I 4026 AA and He I
D_3) has been studied in detail for a total of 14 flares. We have also
estimated the physical parameters of the flaring plasma by using a procedure
which assumes a simplified slab model of flares. All the obtained physical
parameters are consistent with previously derived values for stellar flares,
and the areas - less than 2.3% of the stellar surface - are comparable with the
size inferred for other solar and stellar flares. Finally, we have studied the
relationships between the physical parameters and the area, duration, maximum
flux and energy released during the detected flares.Comment: Latex file with 17 pages, 11 figures. Available at
http://www.ucm.es/info/Astrof/invest/actividad/actividad_pub.html Accepted
for publication in: Astronomy & Astrophysics (A&A
HYDROGEN BALMER CONTINUUM IN SOLAR FLARES DETECTED BY THE INTERFACE REGION IMAGING SPECTROGRAPH
Discovery of 1-(3-{2-[4-(2-Methyl-5-quinolinyl)-1-piperazinyl]ethyl}phenyl)-2-imidazolidinone (GSK163090), a Potent, Selective, and Orally Active 5-HT(1A/B/D) Receptor Antagonist.
In an effort to identify selective drug like pan-antagonists of the 5-HT(1) autoreceptors, studies were conducted to elaborate a previously reported dual acting 5-HT(1) antagonist/SSRI structure. A novel series of compounds was identified showing low intrinsic activities and potent affinities across the 5-HT(1A), 5-HT(1B), and 5-HT(1D) receptors as well as high selectivity against the serotonin transporter. From among these compounds, 1-(3-{2-[4-(2-methyl-5-quinolinyl)-1-piperazinyl]ethyl}phenyl)-2-imidazolidinone (36) was found to combine potent in vivo activity with a strong preclinical developability profile, and on this basis it was selected as a drug candidate with the aim of assessing its potential as a fast-onset antidepressant/anxiolytic