27 research outputs found
The ambiguous transient ASASSN-17hx. A possible nova-impostor
Some transients, although classified as novae based on their maximum and early decline optical spectra, cast doubts on their true nature and whether nova impostors might exist. We monitored a candidate nova which displayed a distinctly unusual light curve at maximum and early decline through optical spectroscopy (3000-10000 \AA, 500<R<100000) complemented with Swift UV and AAVSO optical photometry. We use the spectral line series to characterize the ejecta dynamics, structure, and mass. We found that the ejecta are in free ballistic expansion and structured as typical of classical novae. However, their derived mass is at least an order of magnitude larger than the typical ejecta masses obtained for classical novae. Specifically, we found M910 M independent of the distance for a filling factor =1. By constraining the distance we derived in the range 0.08-0.10, giving a mass 710 M910 M. The nebular spectrum, characterized by unusually strong coronal emission lines, confines the ionizing source energy to the range 20-250 eV, possibly peaking in the range 75-100 or 75-150 eV. We link this source to other slow novae which showed similar behavior and suggest that they might form a distinct physical sub-group. They may result from a classical nova explosion occurring on a very low mass white dwarf or be impostors for an entirely different type of transient
Discovery of a Hot Symbiotic Star in the Cold Antarctic Sky: Symbiotics Are Outliers in SkyMapper uvgriz Photometry
Using near-ultraviolet flux excess and variability from the SkyMapper Southern Sky Survey11 as novel diagnostics to search for symbiotic stars?cool giants accreting onto compact objects, typically white dwarfs (WDs), we report that Hen 3-1768 (≡ASAS J195948?8252.7) is a symbiotic star. It may be an optimal target for continuous monitoring by Antarctic telescopes; at a decl. of −82877, it is now the closest known symbiotic to either geographic pole, and the only known symbiotic more southern than the Small Magellanic Cloud (Belczyński et al. 2000; Akras et al. 2019, ApJS, in press). Figure 1 shows that Hen 3-1768 produced unambiguous emission from Raman O vi 6830,7088 Å, He ii 4686 Å, and other transitions, proving that it is a symbiotic star (e.g., Shore et al. 2014). Comparing to Pickles (1998) template spectra, we preliminarily constrained the giant donor´s spectral type to between K4 and K7, making Hen 3-1768 one of the dozen or so yellow symbiotics with stellar-type infrared (IR) colors currently known (e.g., Baella et al. 2016). The 2MASS IR colors (J − H)0 = 0.82 and (H − Ks)0 = 0.21 (Skrutskie et al. 2006; de-reddened by total Galactic extinction12 ) are consistent with this conclusion (see Figure 1 in Baella et al. 2016).Fil: Lucy, Adrian B.. Columbia University; Estados UnidosFil: Sokoloski, J. L.. Columbia University; Estados UnidosFil: Nuñez, Natalia Edith. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; ArgentinaFil: Wolf, C.. Research School Of Astronomy And Astrophysics, Anu; AustraliaFil: Bohlsen, T.. Mirranook Observatory,; AustraliaFil: Luna, Gerardo Juan Manuel. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin
V618 Sgr: Galactic eclipsing symbiotic nova detected in repeated outbursts
V618 Sgr was previously classified as an R CrB-type variable and later as a
possible symbiotic star. Our study aims to analyse the nature of this target,
which is currently undergoing significant brightening in properties similar to
those of known symbiotic novae. We analyse literature information, photometric
observations, and 35 new optical spectra. Our findings strongly suggest that
V618 Sgr is an eclipsing symbiotic nova currently in outburst. Additionally,
since the star has demonstrated at least two similar brightenings in the past,
we propose that V618 Sgr could be the first known galactic symbiotic nova
observed in repeated outbursts of this type and may host a relatively massive
white dwarf.Comment: 6 pages, 6 figures, 1 table; accepted in MNRA
Early evolution of the extraordinary Nova Del 2013 (V339 Del)
We determine the temporal evolution of the luminosity L(WD), radius R(WD) and
effective temperature Teff of the white dwarf (WD) pseudophotosphere of V339
Del from its discovery to around day 40. Another main objective was studying
the ionization structure of the ejecta. These aims were achieved by modelling
the optical/near-IR spectral energy distribution (SED) using low-resolution
spectroscopy (3500 - 9200 A), UBVRcIc and JHKLM photometry. During the fireball
stage (Aug. 14.8 - 19.9, 2013), Teff was in the range of 6000 - 12000 K, R(WD)
was expanding non-uniformly in time from around 66 to around 300 (d/3 kpc)
R(Sun), and L(WD) was super-Eddington, but not constant. After the fireball
stage, a large emission measure of 1.0-2.0E+62 (d/3 kpc)**2 cm**(-3)
constrained the lower limit of L(WD) to be well above the super-Eddington
value. The evolution of the H-alpha line and mainly the transient emergence of
the Raman-scattered O VI 1032 A line suggested a biconical ionization structure
of the ejecta with a disk-like H I region persisting around the WD until its
total ionization, around day 40. It is evident that the nova was not evolving
according to the current theoretical prediction. The unusual non-spherically
symmetric ejecta of nova V339 Del and its extreme physical conditions and
evolution during and after the fireball stage represent interesting new
challenges for the theoretical modelling of the nova phenomenon.Comment: 14 pages, 9 figures, 3 tables, accepted for Astronomy and
Astrophysic
A Detailed Observational Analysis of V1324 Sco, the Most Gamma-Ray Luminous Classical Nova to Date
It has recently been discovered that some, if not all, classical novae emit
GeV gamma rays during outburst, but the mechanisms involved in the production
of the gamma rays are still not well understood. We present here a
comprehensive multi-wavelength dataset---from radio to X-rays---for the most
gamma-ray luminous classical nova to-date, V1324 Sco. Using this dataset, we
show that V1324 Sco is a canonical dusty Fe-II type nova, with a maximum ejecta
velocity of 2600 km s and an ejecta mass of few
M. There is also evidence for complex shock interactions, including a
double-peaked radio light curve which shows high brightness temperatures at
early times. To explore why V1324~Sco was so gamma-ray luminous, we present a
model of the nova ejecta featuring strong internal shocks, and find that higher
gamma-ray luminosities result from higher ejecta velocities and/or mass-loss
rates. Comparison of V1324~Sco with other gamma-ray detected novae does not
show clear signatures of either, and we conclude that a larger sample of
similarly well-observed novae is needed to understand the origin and variation
of gamma rays in novae.Comment: 26 pages, 13 figure
He II 4686 emission from the massive binary system in Car: constraints to the orbital elements and the nature of the periodic minima
{\eta} Carinae is an extremely massive binary system in which rapid spectrum
variations occur near periastron. Most notably, near periastron the He II
line increases rapidly in strength, drops to a minimum value,
then increases briefly before fading away. To understand this behavior, we
conducted an intense spectroscopic monitoring of the He II
emission line across the 2014.6 periastron passage using ground- and
space-based telescopes. Comparison with previous data confirmed the overall
repeatability of EW(He II ), the line radial velocities, and the
timing of the minimum, though the strongest peak was systematically larger in
2014 than in 2009 by 26%. The EW(He II ) variations, combined
with other measurements, yield an orbital period d. The observed
variability of the EW(He II ) was reproduced by a model in which
the line flux primarily arises at the apex of the wind-wind collision and
scales inversely with the square of the stellar separation, if we account for
the excess emission as the companion star plunges into the hot inner layers of
the primary's atmosphere, and including absorption from the disturbed primary
wind between the source and the observer. This model constrains the orbital
inclination to -, and the longitude of periastron to
-. It also suggests that periastron passage occurred on
d. Our model also reproduced EW(He II )
variations from a polar view of the primary star as determined from the
observed He II emission scattered off the Homunculus nebula.Comment: The article contains 23 pages and 17 figures. It has been accepted
for publication in Ap
The apparent eta Carinae's long-term evolution and the critical role played by the strengthening of P Cygni absorption lines
Over the entire 20th century, Eta Carinae (\ec) has displayed a unique
spectrum, which recently has been evolving towards that of a typical LBV. The
two competing scenarios to explain such evolution are: (1) a dissipating
occulter in front of a stable star or (2) a decreasing mass loss rate of the
star. The first mechanism simultaneously explains why the central star appears
to be secularly increasing its apparent brightness while its luminosity does
not change; why the Homunculus' apparent brightness remains almost constant;
and why the spectrum seen in direct light is becoming more similar to that
reflected from the Homunculus (and which resembles a typical LBV). The second
scenario does not account for these facts and predicts an increase in the
terminal speed of the wind, contrary to observations. In this work, we present
new data showing that the P Cygni absorption lines are secularly strengthening,
which is not the expected behaviour for a decreasing wind-density scenario.
CMFGEN modelling of the primary's wind with a small occulter in front agrees
with observations. One could argue that invoking a dissipating coronagraphic
occulter makes this object even more peculiar than it already appears to be.
However, on the contrary, it solves the apparent contradictions between many
observations. Moreover, by assigning the long-term behaviour to circumstellar
causes and the periodic variations due to binarity, a star more stable after
the 1900s than previously thought is revealed, contrary to the earlier paradigm
of an unpredictable object.Comment: 17 pages, 12 figures, submitted to MNRA
Towards a consistent model of the hot quadruple system HD 93206 = QZ Carin\ae: II. N-body model
HD 93206 is early-type massive stellar system, composed of components
resolved by direct imaging (Ab, Ad, B, C, D) as well as a compact sub-system
(Aa1, Aa2, Ac1, Ac2). Its geometry was already determined on the basis of
extensive photometric, spectroscopic and interferometric observations. However,
the fundamental absolute parameters are still not known precisely enough. We
use an advanced N-body model to account for all mutual gravitational
perturbations among the four close components, and all observational data
types, including: astrometry, radial velocities, eclipse timing variations,
squared visibilities, closure phases, triple products, normalized spectra, and
spectral-energy distribution (SED). The respective model has 38 free
parameters, namely three sets of orbital elements, component masses, and their
basic radiative properties (, , ). We revised the
fundamental parameters of QZ Car as follows. For a model with the nominal
extinction coefficient , the best-fit masses are
, , , , with uncertainties of the order of , and the
system distance . In an alternative model, where
we increased the weights of RV and TTV observations and relaxed the SED
constraints, because extinction can be anomalous with , the
distance is smaller, . This would correspond to
that of Collinder 228 cluster. Independently, this is confirmed by dereddening
of the SED, which is only then consistent with the early-type classification
(O9.7Ib for Aa1, O8III for Ac1). Future modelling should also account for an
accretion disk around Ac2 component.Comment: A&A, submitte
He II λ4686 emission from the massive binary system in η car: constraints to the orbital elements and the nature of the periodic minima
Eta Carinae (η Car) is an extremely massive binary system in which rapid spectrum variations occur near periastron. Most notably, near periastron the He ii λ4686 line increases rapidly in strength, drops to a minimum value, then increases briefly before fading away. To understand this behavior, we conducted an intense spectroscopic monitoring of the He ii λ4686 emission line across the 2014.6 periastron passage using ground- and space-based telescopes. Comparison with previous data confirmed the overall repeatability of the line equivalent width (EW), radial velocities, and the timing of the minimum, though the strongest peak was systematically larger in 2014 than in 2009 by 26%. The EW variations, combined with other measurements, yield an orbital period of 2022.7 ±0.3 days. The observed variability of the EW was reproduced by a model in which the line flux primarily arises at the apex of the wind-wind collision and scales inversely with the square of the stellar separation, if we account for the excess emission as the companion star plunges into the hot inner layers of the primary's atmosphere, and including absorption from the disturbed primary wind between the source and the observer. This model constrains the orbital inclination to 135°-153°, and the longitude of periastron to 234°-252°. It also suggests that periastron passage occurred on days). Our model also reproduced EW variations from a polar view of the primary star as determined from the observed He ii λ4686 emission scattered off the Homunculus nebula.Facultad de Ciencias Astronómicas y Geofísica