1,396 research outputs found
Discovery of an Unusual High State in the Long-Term Light Curve of AM Herculis
The magnetic cataclysmic variable archetype AM Herculis is known to display high and low photometric
states, in which it switches from Roche lobe-overflow accretion (high) to wind accretion (low). The origin of
this behavior is still unknown (although it is believed to be linked to the presence of starspots located near the L1
point on the secondary star), as are the specifics of duration and frequency of the two states. During the recent (2007)
transition from an exceptionally long period of persistent low state behavior back to the high state, AM Her entered a
short (~2 month) intermediate state, in which its brightness remained at ~0.5 mag fainter than the normal high-state
level. Within the recorded history of AM Her, this was the first time the system displayed such behavior. Using data
from the American Association of Variable Star Observers, we compare the characteristics of this faint high state to
the subsequent normal high state, and a prior "failed high state," in which the system briefly reached a brightness
level near that of the normal high state before rapidly returning to the low state. The distribution of magnitude
values attained during each state, along with the shapes of the orbital light curve in each state, support a scenario
in which the specific configuration of the accretion flow between the secondary star and the white dwarf must be
rapidly established at the end of a low state, even before the mass transfer rate has reached the normal high-state
level
QU Carinae: a SNeIa progenitor?
Optical spectra obtained in 2006-07 of the nova-like cataclysmic variable QU
Car are studied for radial velocities, line profiles, and line identifications.
We are not able to confirm the reported 10.9 hr orbital period from 1982,partly
because our sampling is not ideal for this purpose and also, we suspect,
because our radial velocities are distorted by line profile changes due to an
erratic wind. P-Cygni profiles are found in several of the emission lines,
including those of C IV. Carbon lines are abundant in the spectra, suggesting a
carbon enrichment in the doner star. The presence of [O III] 5007\AA and [N II]
6584\AA is likely due to a diffuse nebula in the vicinity of the system.
The wind signatures in the spectra and the presence of nebular lines are in
agreement with the accretion wind evolution scenario that has been suggested to
lead to SNeIa. We argue that QU Car is a member of the V Sge subclass of CVs,
and a possible SNeIa progenitor. It is shown that the recent light curve of QU
Car has ~1 mag low states, similar to the light curve of V Sge, strengthening
the connection of QU Car with V Sge stars, supersoft x-ray sources, and SNeIa
progenitors.Comment: Accepted in the Astronomical Journal. 11 pages, 3 tables, 5 figure
What’s Cool About Hot Stars? Cataclysmic Variables in the Mid-Infrared
We review recent results from mid-infrared observations of cataclysmic variables with the Spitzer Space Telescope. In general, these observations have revealed mid-infrared excesses, above the level expected from the stellar and accretion components, in numerous systems. This excess can be modeled as originating from circumstellar and/or circumbinary dust. We present an overview of spectral energy distributions spanning the ultraviolet to the mid-infrared, as well as mid-infrared light curves, of disk-accreting and magnetic cataclysmic variables. Physically realistic models constructed to reproduce these data indicate that the mid-infrared luminosity of many cataclysmic variables is dominated by emission from warm (T < 2000 K) dust. The presence and characteristics of dust in cataclysmic variables has potentially important implications for the secular evolution scenario for interacting binary stars
Observations of V592 Cas -— an Outflow at Optical Wavelengths
We present new red optical spectra of V592 Cas aimed at exploring the properties of the outflow of this system in a spectral region where the underlying white dwarf and the accretion disk do not contribute significantly to the observed absorption components of the Hα and He I line profiles. We use the Hα emission line to study the wind, which appears as pronounced blueshifted P Cygni absorption troughs whose low velocity end contaminates the blue side of the emission line profile. The wind appears to be episodic in nature, with multiple events reaching velocities of 5000 km s^–1 in Hα. Similar (but weaker) wind signatures appear in the He I 5876 Å line but are absent in He I 6678 Å. Our data suggest that during wind episodes the wind is phase dependent and is visible for half of the orbit of the system. Considering that V592 Cas is viewed almost face-on, the symmetry axis of the outflow cannot be orthogonal to the disk and/or the outflow must have some other inherent asymmetry in outflow geometry. A possible origin of the wind is in a disk hotspot, either at the initial impact point of the accretion stream on the disk edge or as a result of disk overflow (similar to SW Sextantis stars). Simultaneous optical photometry during one night of our spectroscopic observations indicate that there is no clear relationship between the optical brightness variations and the strength of the outflow in this system
Accretion and activity on the post-common-envelope binary RR~Cae
Current scenarios for the evolution of interacting close binaries - such as
cataclysmic variables (CVs) - rely mainly on our understanding of low-mass star
angular momentum loss (AML) mechanisms. The coupling of stellar wind with its
magnetic field, i.e., magnetic braking, is the most promising mechanism to
drive AML in these stars. There are basically two properties driving magnetic
braking: the stellar magnetic field and the stellar wind. Understanding the
mechanisms that drive AML therefore requires a comprehensive understanding of
these two properties. RRCae is a well-known nearby (d=20pc) eclipsing DA+M
binary with an orbital period of P=7.29h. The system harbors a metal-rich cool
white dwarf (WD) and a highly active M-dwarf locked in synchronous rotation.
The metallicity of the WD suggests that wind accretion is taking place, which
provides a good opportunity to obtain the mass-loss rate of the M-dwarf
component. We analyzed multi-epoch time-resolved high-resolution spectra of
RRCae in search for traces of magnetic activity and accretion. We selected a
number of well-known activity indicators and studied their short and long-term
behavior. Indirect-imaging tomographic techniques were also applied to provide
the surface brightness distribution of the magnetically active M-dwarf, and
reveals a polar feature similar to those observed in fast-rotating solar-type
stars. The blue part of the spectrum was modeled using a atmosphere model to
constrain the WD properties and its metal enrichment. The latter was used to
improve the determination of the mass-accretion rate from the M-dwarf wind. The
presence of metals in the WD spectrum suggests that this component arises from
accretion of the M-dwarf wind. A model fit to the WD gives Teff=(7260+/-250)K
and logg=(7.8+/-0.1) dex with a metallicity of =(-2.8+/-0.1)dex,
and a mass-accretion rate of dotMacc=(7+/-2)x1e-16Msun/yr.Comment: 14 pages, 7 Figures, 6 Table
The Amazing Old Nova Q Cygni: A Far Ultraviolet Synthetic Spectral Analysis
Q Cygni (Nova Cygni 1876) is the third oldest old novae (after WY Sge and
V841 Oph) with a long orbital period of 10.08 hours and spectroscopic
peculiarities in the optical including the presence of variable wind outflow
revealed by optical P Cygni profiles in the HeI lines and H alpha beta (Kafka
et al. 2003). We have carried out a synthetic spectral analysis of a far
ultraviolet IUE archival spectrum of Q Cygni using our optically thick, steady
state, accretion disk models and model white dwarf photospheres. We find that
the accretion light of a luminous accretion disk dominates the FUV flux of the
hot component with a rate of accretion 2-3 1.E-9 Msun/yr. We find that Q Cygni
lies at a distance of 741 \pm 110 pc . The implications of our results for
theoretical predictions for old novae are presented.Comment: PASP, August 201
Apparent multiple Delta m^2_32 in muon anti-neutrino and muon neutrino survival oscillations from non-standard interaction matter effect
Neutrinos propagating through matter may participate in forward coherent
neutral-current-like scattering arising from non-standard interactions as well
as from the Mikheyev-Smirnov-Wolfenstein matter potential . We show that
at fixed long baselines through matter of constant density, the non-standard
interaction potential can contribute an additional
term to the oscillation phase whose sign differs for \anumu versus \numu
propagation in matter. Its presence can cause different apparent
to be erroneously inferred on the basis of oscillations in vacuum, with values
lying above (for \anumu) or below (for \numu) the actual
for the case where is predominantly real-valued and of
sign opposite to . An NSI scenario invoking only
is shown to be capable of accounting for a disparity
recently reported between oscillation survival for \anumu and \numu fluxes
measured at by the MINOS experiment. Implications for mantle
traversal by atmospheric neutrinos are examined. The NSI matter potential with
non-maximal mixing could evade conventional atmospheric neutrino analyses which
do not distinguish \numu from \anumu on an event-by-event basis.Comment: 7 pages, 5 figures. Accepted for publication in Physical Review
Orbit-resolved photometry and echelle spectroscopy of the cataclysmic variable ST LMi during a 2007 high state
We present high-resolution echelle spectra and contemporaneous photometry of the polar ST LMi during a high state in 2007 March. Emission lines at Hα, He I λ5876, and He I λ7065 show similar line profiles over orbital phase and have narrow and broad components. These profile changes with phase are very similar to those reported in earlier high-state studies of ST LMi. The radial velocity curves from double Gaussian fits to the line profiles are interpreted as two crossing curves, neither of which is coincident with the orbital motion of the secondary star. We attribute one component to infall motions near the white dwarf and the other to a gas streaming along magnetic field lines connecting the two stars
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