71 research outputs found
The secondary minimum in YY Her: Evidence for a tidally distorted giant
We present and analyze quiescent UBVRI light curves of the classical
symbiotic binary YY Her. We show that the secondary minimum, which is clearly
visible only in the quiescent VRI light curves, is due to ellipsoidal
variability of the red giant component. Our simple light curve analysis, by
fitting of the Fourier cosine series, resulted in a self-consistent
phenomenological model of YY Her, in which the periodic changes can be
described by a combination of the ellipsoidal changes and a sinusoidal changes
of the nebular continuum and line emission.Comment: 5 pages, 2 figures, to appear in Astronomy & Astrophysic
DASCH Discovery of A Possible Nova-like Outburst in A Peculiar Symbiotic Binary
We present photometric and spectroscopic observations of a peculiar variable
(designated DASCH J075731.1+201735 or J0757) discovered from our DASCH project
using the digitized Harvard College Observatory archival photographic plates.
It brightened by about 1.5 magnitudes in B within a year starting in 1942, and
then slowly faded back to its pre-outburst brightness from 1943 to the 1950s.
The mean brightness level was stable before and after the outburst, and
ellipsoidal variations with a period of days are seen,
suggesting that the star is tidally distorted. Radial-velocity measurements
indicate that the orbit is nearly circular () with a
spectroscopic period that is the same as the photometric period. The binary
consists of a M0III star, and a
companion, very likely a white dwarf (WD). Unlike other symbiotic binaries,
there is no sign of emission lines or a stellar wind in the spectra. With an
outburst timescale of ~10 years and estimated B band peak luminosity M_B~0.7,
J0757 is different from any other known classic or symbiotic novae. The most
probable explanation of the outburst is Hydrogen shell-burning on the WD,
although an accretion-powered flare cannot be ruled out.Comment: 12 pages, 6 figures, accepted for publication in Ap
The remarkable properties of the symbiotic star AE Circinus
We present new optical spectroscopy and photometry, 2MASS infrared
observations and 24 years of combined AAVSO and AFOEV photometry of the
symbiotic star candidate \ae. The long-term light curve is characterized by
outbursts lasting several years and having a slow decline of mag/day. The whole range of variability of the star in the band is
about 4 magnitudes. The periodogram of the photometric data reveals strong
signals at 342 and 171 days. The presence of the emission feature at
6830 \AA at minimum and the detection of absorption lines of a
K5 type star confirm the symbiotic classification and suggest that AE Cir is a
new member of the small group of s-type yellow symbiotic stars. We estimate a
distance of 9.4 kpc. Our spectrum taken at the high state shows a much flatter
spectral energy distribution, the disappearance of the 6830 \AA
emission feature and the weakness of the He II 4686 emission relative to the
Balmer emission lines. Our observations indicate the presence of emission line
flickering in time scales of minutes in 2001. The peculiar character of \ae is
revealed in the visibility of the secondary star at the high and low state, the
light curve resembling a dwarf nova superoutburst and the relatively short low
states. The data are hard to reconciliate with standard models for symbiotic
star outbursts.Comment: accepted for publication in MNRAS, 7 figure
Synthetic Spectral Analysis of the Hot Component in the S-Type Symbiotic Variable EG Andromeda
We have applied grids of NLTE high gravity model atmospheres and optically
thick accretion disk models for the first time to archival IUE and FUSE spectra
of the S-type symbiotic variable EG And taken at superior spectroscopic
conjunction when Rayleigh scattering should be minimal and the hot component is
viewed in front of the red giant. For EG And's widely accepted, published hot
component mass, orbital inclination and distance from the Hipparcos parallax,
we find that hot, high gravity, NLTE photosphere model fits to the IUE spectra
yield distances from the best-fitting models which agree with the Hipparcos
parallax distance but at temperatures substantially lower than the modified
Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same
orbital phase as the IUE spectra yield the same temperature as the IUE
temperature (50,000K). However, for the same hot component mass, inclination
and parallax-derived distance, accretion disk models at moderately high
inclinations, with accretion rates to 1\times 10^{-9} M_{\sun}/yr for white dwarf masses M_{wd} = 0.4
M_{\sun} yield distances grossly smaller than the distance from the Hipparcos
parallax. Therefore, we rule out an accretion disk as the dominant source of
the FUV flux. Our findings support a hot bare white dwarf as the dominant
source of FUV flux.Comment: AJ, Oct. 200
A silicate disk in the heart of the Ant
We aim at getting high spatial resolution information on the dusty core of
bipolar planetary nebulae to directly constrain the shaping process. Methods:
We present observations of the dusty core of the extreme bipolar planetary
nebula Menzel 3 (Mz 3, Hen 2-154, the Ant) taken with the mid-infrared
interferometer MIDI/VLTI and the adaptive optics NACO/VLT. The core of Mz 3 is
clearly resolved with MIDI in the interferometric mode, whereas it is
unresolved from the Ks to the N bands with single dish 8.2 m observations on a
scale ranging from 60 to 250 mas. A striking dependence of the dust core size
with the PA angle of the baselines is observed, that is highly suggestive of an
edge-on disk whose major axis is perpendicular to the axis of the bipolar
lobes. The MIDI spectrum and the visibilities of Mz 3 exhibit a clear signature
of amorphous silicate, in contrast to the signatures of crystalline silicates
detected in binary post-AGB systems, suggesting that the disk might be
relatively young. We used radiative-transfer Monte Carlo simulations of a
passive disk to constrain its geometrical and physical parameters. Its
inclination (74 degrees 3 degrees) and position angle (5 degrees 5
degrees) are in accordance with the values derived from the study of the lobes.
The inner radius is 9 1 AU and the disk is relatively flat. The dust mass
stored in the disk, estimated as 1 x 10-5Msun, represents only a small fraction
of the dust mass found in the lobes and might be a kind of relic of an
essentially polar ejection process
Understanding Needs, Identifying Opportunities: ICT in the View of Universal Design
This article provides food for thoughts elaborated by peer researchers who, basing on their studies and on current literature on relationships between Universal Design (UD) and Information and Communication Technologies (ICT), wish to share few key issues related to the challenges offered by the involvement of final users in designing product and services. Referring to approaches from different disciplines, key questions will be highlighted on which a debate could start, focused on the issue of promoting inclusion and how a close relationship among these different areas of knowledge can contribute to bridge the gap between the potential of new technologies and the real and diversified need by persons. Thus, actively contributing toward the empowerment of the community of belonging
Catalog of 93 Nova Light Curves: Classification and Properties
We present a catalog of 93 very-well-observed nova light curves. The light
curves were constructed from 229,796 individual measured magnitudes, with the
median coverage extending to 8.0 mag below peak and 26% of the light curves
following the eruption all the way to quiescence. Our time-binned light curves
are presented in figures and as complete tabulations. We also calculate and
tabulate many properties about the light curves, including peak magnitudes and
dates, times to decline by 2, 3, 6, and 9 magnitudes from maximum, the time
until the brightness returns to quiescence, the quiescent magnitude, power law
indices of the decline rates throughout the eruption, the break times in this
decline, plus many more properties specific to each nova class. We present a
classification system for nova light curves based on the shape and the time to
decline by 3 magnitudes from peak (t3). The designations are S for smooth light
curves (38% of the novae), P for plateaus (21%), D for dust dips (18%), C for
cusp-shaped secondary maxima (1%), O for quasi-sinusoidal oscillations
superposed on an otherwise smooth decline (4%), F for flat-topped light curves
(2%), and J for jitters or flares superposed on the decline (16%). Our
classification consists of this single letter followed by the t3 value in
parentheses; so for example V1500 Cyg is S(4), GK Per is O(13), DQ Her is
D(100), and U Sco is P(3).Comment: Astronomical Journal, in press, 19 figures, 73 page
A measurement of the broad-band spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications
We report on results of a target of opportunity observation of the X-ray
transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field
Instruments (0.1-200 keV) of the SAX satellite. The measured spectrum is a
power law with a photon index of ~1.7 modified by an ultrasoft X-ray excess and
a high-energy cutoff above ~100 keV. The soft excess is consistent with a
blackbody with temperature of ~40 eV and a low flux, while the cut-off power
law is well fitted by thermal Comptonization in a plasma with an electron
temperature of 100 keV and an optical depth of order of unity. Consistent with
the weakness of the blackbody, Compton reflection is weak. Though the data are
consistent with various geometries of the hot and cold phases of the accreting
gas, we conclude that a hot accretion disk is the most plausible model. The
Eddington ratio implied by recent estimates of the mass and the distance is
about 10^{-3}, which may indicate that advection is probably not the dominant
cooling mechanism. We finally suggest that the reflecting medium has a low
metallicity, consistent with location of the system in the halo.Comment: 14 pages, 8 figures, accepted for publication in The Astrophysical
Journa
Catching a nova X-ray/UV flash in the visible? Early spectroscopy of the extremely slow Nova Velorum 2022 (Gaia22alz)
We present early spectral observations of the very slow Galactic nova
Gaia22alz, over its gradual rise to peak brightness that lasted 180 days.
During the first 50 days, when the nova was only 3--4 magnitudes above its
normal brightness, the spectra showed narrow (FWHM 400 km s)
emission lines of H Balmer, He I, He II, and C IV, but no P Cygni absorption. A
few weeks later, the high-excitation He II and C IV lines disappeared, and P
Cygni profiles of Balmer, He I, and eventually Fe II lines emerged, yielding a
spectrum typical of classical novae before peak. We propose that the early
spectra of Gaia22alz are produced in the white dwarf's envelope or accretion
disk, reprocessing X-ray and ultraviolet emission from the white dwarf after a
dramatic increase in the rate of thermonuclear reactions, during a phase known
as the ``early X-ray/UV flash''. If true, this would be one of the rare times
that the optical signature of the early X-ray/UV flash has been detected. While
this phase might last only a few hours in other novae and thus be easily
missed, it was possible to detect in Gaia22alz due to its very slow and gradual
rise and thanks to the efficiency of new all-sky surveys in detecting
transients on their rise. We also consider alternative scenarios that could
explain the early spectral features of Gaia22alz and its unusually slow rise.Comment: 20 pages, 12 figures, 2 tables. Submitted to MNRA
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