2,599 research outputs found
On the narrow emission line components of the LMC novae 2004 (YY Dor) and 2009a
We present early decline spectra of the two Large Magellanic Cloud novae: LMC
2004 (YY Dor) and LMC 2009a and discuss their spectral an line profile
evolution with special emphasis on the existence and appearance of a sharp
component. We show that the narrow component that characterizes the emission
lines in the maximum spectra of nova LMC 2004 originates in the ejecta. The
HeII 4686 A, narrow emission which appears at the onset of the nebular phase in
both novae is somewhat controversial. Our observations suggest that the
corresponding line forming region is physically separated from the rest of the
ejecta (the broad line region) and environmentally different. However, the lack
of late time observations covering the super-soft source (SSS) phase, the
post-SSS phase and the quiescence state does not allow to securely establish
any non-ejecta origin/contribution as, instead, in the case of U Sco and KT
Eri.Comment: 11 pages, 9 figures. Accepted for publication in Astronomy and
Astrophysics on Aug 13 201
Post-outburst spectra of a stellar-merger remnant of V1309 Scorpii: from a twin of V838 Monocerotis to a clone of V4332 Sagittarii
We present optical and infrared spectroscopy of V1309 Sco, an object that
erupted in 2008 in a stellar-merger event. During the outburst, V1309 Sco
displayed characteristics typical of red transients, a class of objects similar
to V838 Mon. Our observations were obtained in 2009 and 2012, i.e. months and
years after the eruption of V1309 Sco, and illustrate severe changes in the
remnant, mainly in its circumstellar surroundings. In addition to atomic gas
observed in earlier epochs, we identified molecular bands of TiO, VO, HO,
ScO, AlO, and CrO. The infrared bands of CrO we analyse are the first
astronomical identification of the features. Over the whole period covered by
our data, the remnant was associated with a cool (1000 K) outflow
with a terminal velocity of about 200 km/s. Signatures of warmer atomic gas,
likely to be still dissipating the energy of the 2008 outburst, dramatically
decreased their brightness between 2009 and 2012. In addition, the source of
optical continuum disappeared sometime before 2012, likely owing to the
formation of new dust. The final stage of V1309 Sco's evolution captured by our
spectra is an object remarkably similar to an older red transient, V4332 Sgr.
In addition to providing a detailed view on the settling of the eruptive
object, the observations presented here reinforce the conclusion that all the
Galactic red transients are a manifestation of the same phenomenon, i.e. a
stellar merger. The late spectra of V1309 Sco also suggest peculiarities in the
chemical composition of the remnant, which still need to be explored.Comment: to appear in A&
Spectroscopic diagnostics of dust formation and evolution in classical nova ejecta
A fraction of classical novae form dust during the early stages of their
outbursts. The classical CO nova V5668 Sgr (Nova Sgr. 2015b) underwent a deep
photometric minimum about 100 days after outburst that was covered across the
spectrum. A similar event was observed for an earlier CO nova, V705 Cas (Nova
Cas 1993) and a less optically significant event for the more recent CO nova
V339 Del (Nova Del 2013). This study provides a "compare and contrast" of these
events to better understand the very dynamical event of dust formation. We show
the effect of dust formation on multiwavelength high resolution line profiles
in the interval 1200\AA\ - 9200\AA\ using a biconical ballistic structure that
has been applied in our previous studies of the ejecta. We find that both V5668
Sgr and V339 Del can be modeled using a grey opacity for the dust, indicating
fairly large grains (at least 0.1 micron) and that the persistent asymmetries
of the line profiles in late time spectra, up to 650 days after the event for
V5668 Sgr and 866 days for V339 Del, point to the survival of the dust well
into the transparent, nebular stage of the ejecta evolution. This is a general
method for assessing the properties of dust forming novae well after the
infrared is completely transparent in the ejecta.Comment: 15 pages 14 figures, accepted for publication in A&A, 2018 June 2
Gemini/GMOS search of massive binaries in the ionizing cluster of 30 Dor
If binaries are common among massive stars, it will have important
consequences for the derivation of fundamental properties like the cluster age,
IMF and dynamical mass. Making use of the multiplexing facilities of Gemini
Multi Object Spectrograph (GMOS) we were able to investigate the presence of
binary stars within the ionising cluster of 30 Doradus. From a seven epochs
observing campaign at Gemini South we detect a binary candidate rate of about
50%, which is consistent with an intrinsic 100% binary rate among massive
stars. We find that single epoch determinations of the velocity dispersion give
values around 30 km/s . After correcting the global velocity dispersion for the
binary orbital motions, the true cluster velocity dispersion is 8.3 km/s. This
value implies a virial mass of about 4.5E5 Msun or 8 percent of the mass
calculated using the single epoch value. The binary corrected virial mass
estimate is consistent with photometric mass determinations thus suggesting
that NGC 2070 is a firm candidate for a future globular cluster.Comment: 9 pages, accepted at Astronomical Journa
On the M_V -- Inclination Relationship for Nova-like Variables
Using a sample of Nova-like stars from the Ritter and Kolb catalog, we
examine the relationship between their Gaia determined absolute magnitude and
the inclination of the binary system. Webbink et al. (1987) derived a
relationship between these two variables that provides a good fit and allows
differentiation between (and possibly M_{WD}) as a function of
inclination. We show that the spread in M_V, at a given i, is dominated by the
mass transfer rate with only a small dependence on the white dwarf mass. The
validated relation shows that present-day theoretical population studies of
cataclysmic variables as well as model fits to observational data yield mass
transfer rates and white dwarf masses consistent with the Gaia derived M_V for
the nova-like stars.Comment: Accepted to A
Revisiting V1309 Sco 2008 outburst spectra. Observational evidence for theoretical modeling of stellar mergers
CONTEXT: V1309 Sco is the only certain noncompact stellar merger, due to its
indisputable preoutburst light curve matching that of a contact binary of
almost equal mass stars. Therefore, anything that can be deduced from the
existing observations serves as benchmark constraints for models. AIMS: We
present some observational evidences to guide future hydrodynamical simulations
and common envelope studies. METHODS: Using archive spectra taken at high and
mid spectral resolution during the V1309 Sco outburst and late decline,
together with the inferential methods we developed to study nova ejecta through
panchromatic high resolution spectroscopic follow ups, we constrain the
physical state, structure, dynamics and geometry of the transient originated in
the stellar merger. RESULTS: We found that the emitted spectra arise from two
distinct contributions: matter expelled during the 2008 outburst and
circumbinary gas produced during historic mass loss episodes. These two
components likely have orthogonal geometry with the 2008 mass loss displaying a
dust-laden bipolar ejecta produced by a time limited rapidly accelerating wind
and the circumbinary gas having a donut-like shape. A central source powers
them both, having produced a fluorescent light pulse, but we cannot precisely
determine the time it started or its spectral energy distribution. We can,
however, place its upper energy cutoff at about 54 eV and the bulk of its
emission at 20 eV. We also know that the central source turned off within
months from the outburst and before the ejecta turned optically thin.Comment: 11 pages, 9 figures (png format). Accepted for publication in A&
Welfare and Distributional Effects of Road Pricing Schemes for Metropolitan Washington, DC
Economists have long advocated congestion pricing as an efficient way of allocating scarce roadway capacity. However, with a few exceptions, congestion tolls are rarely used in practice and strongly opposed by the public and elected officials. Although high implementation costs and privacy issues are alleviated as appropriate technologies are developed, the concerns that congestion pricing will adversely affect low-income travelers remain. In this paper, we use a strategic transportation planning model calibrated for the Washington, DC, metropolitan area to compare the welfare and distributional effects of three pricing schemes: value pricing (HOT lanes), limited congestion pricing, and comprehensive congestion pricing. We find that social welfare gains from HOT lanes amount to three-quarters of those from the comprehensive road pricing. At the same time, a HOT lanes policy turns out to be much more equitable than other road pricing schemes, with all income groups strictly benefiting even before the toll revenue is recycled.traffic congestion, congestion pricing, value pricing, HOT lanes, HOV lanes
Signatures of the non-Maxwellian -distributions in optically thin line spectra. II. Synthetic Fe XVII--XVIII X-ray coronal spectra and predictions for the Marshall Grazing-Incidence X-ray Spectrometer (MaGIXS)
We investigated the possibility of diagnosing the degree of departure from
the Maxwellian distribution using the Fe XVII - Fe XVIII spectra originating in
plasmas in collisional ionization equilibrium, such as in the cores of solar
active regions or microflares. The original collision strengths for excitation
are integrated over the non-Maxwellian electron -distributions
characterized by a high-energy tail. Synthetic X-ray emission line spectra were
calculated for a range of temperatures and . We focus on the 6-24 A
spectral range to be observed by the upcoming Marshall Grazing-Incidence X-ray
Spectrometer MaGIXS. We find that many line intensity ratios are sensitive to
both and . Best diagnostic options are provided if a ratio
involving both Fe XVII and Fe XVIII is combined with another ratio involving
lines formed within a single ion. The sensitivity of such diagnostics to
is typically a few tens of per cent. Much larger sensitivity, of about
a factor of two to three, can be obtained if the Fe XVIII 93.93 A line observed
by SDO/AIA is used in conjuction with the X-ray lines. We conclude that the
MaGIXS instrument is well-suited for detection of departures from the
Maxwellian distribution, especially in active region cores.Comment: Astronomy & Astrophysics, accepte
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