26,910 research outputs found
Evidence for T Tauri-like emission in the EXor V1118 Ori from near-IR and X-ray data
We present a near-IR study of the EXor variable V1118 Ori, performed by
following a slightly declining phase after a recent outburst. In particular,
the near-IR (0.8 - 2.3 micron) spectrum, obtained for the first time, shows a
large variety of emission features of the HI and HeI recombination and CO
overtone. By comparing the observed spectrum with a wind model, a mass loss
rate value is derived along with other parameters whose values are typical of
an accreting T Tauri star. In addition, we have used X-ray data from the XMM
archive, taken in two different epochs during the declining phase monitored in
IR. X-ray emission (in the range 0.5 - 10 keV) permits to derive several
parameters which confirm the T Tauri nature of the source. In the near-IR the
object maintains a low visual extinction during all the activity phases,
confirming that variable extinction does not contribute to brightness
variations. The lack of both a significant amount of circumstellar material and
any evidence of IR cooling from collimated jet/outflow driven by the source,
indicates that, at least this member of the EXor class, is in a late stage of
the Pre-Main Sequence evolution. In the X-ray regime, an evident fading is
present, detected in the post-outburst phase, that cannot be reconciled with
the presence of any absorbing material. This circumstance, combined with the
persistence (in the pre- and post-outburst phases) of a temperature component
at about 10 MK, suggests that accretion has some influence in regulating the
coronal activity
Chemical tracers in proto-brown dwarfs: CO, ortho-HCO, para-HCO, HCO, CS observations
We present a study of the CO isotopologues and the high-density tracers
HCO, HCO, and CS in Class 0/I proto-brown dwarfs (proto-BDs). We
have used the IRAM 30m telescope to observe the CO (2-1), CO
(2-1), CO (2-1), CO (2-1), HCO (3-2), HCO (3-2), and
CS (5-4) lines in 7 proto-BDs. The hydrogen column density for the proto-BDs
derived from the CO gas emission is 2-15 times lower than that derived
from the dust continuum emission, indicating CO depletion from the gas-phase.
The mean HCO ortho-to-para ratio is 3 for the proto-BDs and
indicates gas-phase formation for HCO. We have investigated the
correlations in the molecular abundances between the proto-BDs and protostars.
Proto-BDs on average show a factor of 2 higher ortho-to-para HCO
ratio than the protostars. Possible explanations include a difference in the
HCO formation mechanism, spin-selective photo-dissociation,
self-shielding effects, or different emitting regions for the ortho and para
species. There is a tentative trend of a decline in the HCO and HCO
abundances with decreasing bolometric luminosity, while the CS and CO
abundances show no particular difference between the proto-BDs and protostars.
These trends reflect the scaled-down physical structures for the proto-BDs
compared to protostars and differences in the peak emitting regions for these
species. The CO isotopologue is detected in all of the proto-BDs as well
as the more evolved Class Flat/Class II BDs in our sample, and can probe the
quiescent gas at both early and late evolutionary stages.Comment: Accepted in MNRAS. arXiv admin note: text overlap with
arXiv:1809.1016
Where Are the Binaries? Results of a Long-Term Search for Radial Velocity Binaries in Proto-Planetary Nebulae
We present the results of an expanded, long-term radial velocity search (25
yrs) for evidence of binarity in a sample of seven bright proto-planetary
nebulae (PPNe). The goal is to investigate the widely-held view that the
bipolar or point-symmetric shapes of planetary nebulae (PNe) and PPNe are due
to binary interactions. Observations from three observatories were combined
from 2007-2015 to search for variations on the order of a few years and then
combined with earlier observations from 1991-1995 to search for variations on
the order of decades. All seven show velocity variations due to periodic
pulsation in the range of 35-135 days. However, in only one PPN, IRAS
22272+5435, did we find even marginal evidence found for multi-year variations
that might be due to a binary companion. This object shows
marginally-significant evidence of a two-year period of low semi-amplitude
which could be due to a low-mass companion, and it also displays some evidence
of a much longer period of >30 years. The absence of evidence in the other six
objects for long-period radial velocity variations due to a binary companion
sets significant constraints on the properties of any undetected binary
companions: they must be of low mass, 30 years.
Thus the present observations do not provide direct support for the binary
hypothesis to explain the shapes of PNe and PPNe and severely constrains the
properties of any such undetected companions.Comment: 28 pages, 5 figure
Binary central stars of planetary nebulae
This paper reviews our knowledge on binary central stars of planetary nebulae
and presents some personal opinions regarding their evolution. Three types of
interactions are distinguished: type I, where the binary companion induces the
mass loss; type II, where it shapes the mass loss but does not enhance it; type
III, where a wide orbit causes the centre of mass to move, leading to a spiral
embedded in the wind. Surveys for binary central stars are discussed, and the
separations are compared to the distribution for binary post-AGB stars. The
effect of close binary evolution on nebular morphology is discussed.
Post-common-envelope binaries are surrounded by thin, expanding disks, expelled
in the orbital plane. Wider binaries give rise to much thicker expanding torii.
Type I binary evolution predicts a wide distribution of masses of central
stars, skewed towards low masses. Comparison with observed mass distributions
suggests that this is unlikely to be the only channel leading to the formation
of a planetary nebula. A new sample of compact Bulge nebulae shows about 40% of
nebulae with binary-induced morphologies.Comment: Invited review, in 'Evolution and chemistry of symbiotic stars and
related objects', Wierzba, August 2006. To appear in Baltic Astronom
Nucleosynthesis in Type I X-ray Bursts
Type I X-ray bursts are thermonuclear explosions that occur in the envelopes
of accreting neutron stars. Detailed observations of these phenomena have
prompted numerous studies in theoretical astrophysics and experimental nuclear
physics since their discovery over 35 years ago. In this review, we begin by
discussing key observational features of these phenomena that may be sensitive
to the particular patterns of nucleosynthesis from the associated thermonuclear
burning. We then summarize efforts to model type I X-ray bursts, with emphasis
on determining the nuclear physics processes involved throughout these bursts.
We discuss and evaluate limitations in the models, particularly with regard to
key uncertainties in the nuclear physics input. Finally, we examine recent,
relevant experimental measurements and outline future prospects to improve our
understanding of these unique environments from observational, theoretical and
experimental perspectives.Comment: Accepted by Prog. Part. Nucl. Phys., 45 pages, 14 figure
The Origin and Shaping of Planetary Nebulae: Putting the Binary Hypothesis to the Test
Planetary nebulae (PNe) are circumstellar gas ejected during an intense
mass-losing phase in the the lives of asymptotic giant branch stars. PNe have a
stunning variety of shapes, most of which are not spherically symmetric. The
debate over what makes and shapes the circumstellar gas of these evolved,
intermediate mass stars has raged for two decades. Today the community is
reaching a consensus that single stars cannot trivially manufacture PNe and
impart to them non spherical shapes and that a binary companion, possibly even
a sub-stellar one, might be needed in a majority of cases. This theoretical
conjecture has however not been tested observationally. In this review we
discuss the problem both from the theoretical and observational standpoints,
explaining the obstacles that stand in the way of a clean observational test
and ways to ameliorate the situation. We also discuss indirect tests of this
hypothesis and its implications for stellar and galactic astrophysics.Comment: 28 pages of text. 4 tables 9 figures. Accepted by PASP Review
The Rate of Type Ia Supernovae at High Redshift
We derive the rates of Type Ia supernovae (SNIa) over a wide range of
redshifts using a complete sample from the IfA Deep Survey. This sample of more
than 100 SNIa is the largest set ever collected from a single survey, and
therefore uniquely powerful for a detailed supernova rate (SNR) calculation.
Measurements of the SNR as a function of cosmological time offer a glimpse into
the relationship between the star formation rate (SFR) and Type Ia SNR, and may
provide evidence for the progenitor pathway. We observe a progressively
increasing Type Ia SNR between redshifts z~0.3-0.8. The Type Ia SNR
measurements are consistent with a short time delay (t~1 Gyr) with respect to
the SFR, indicating a fairly prompt evolution of SNIa progenitor systems. We
derive a best-fit value of SFR/SNR 580 h_70^(-2) M_solar/SNIa for the
conversion factor between star formation and SNIa rates, as determined for a
delay time of t~1 Gyr between the SFR and the Type Ia SNR. More complete
measurements of the Type Ia SNR at z>1 are necessary to conclusively determine
the SFR--SNR relationship and constrain SNIa evolutionary pathways.Comment: 37 pages, 9 figures, accepted for publication in Astrophysical
Journal. Figures 7-9 correcte
- …