234 research outputs found
Distances of Stars by mean of the Phase-lag Method
Variable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an
optically thick circumstellar envelope that emit strong OH 1612 MHz emission.
They are commonly observed throughout the Galaxy but also in the LMC and SMC.
Hence, the precise inference of the distances of these stars will ultimately
result in better constraints on their mass range in different metallicity
environments. Through a multi-year long-term monitoring program at the Nancay
Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at
the eMERLIN and JVLA to measure precisely the angular diameter of the
envelopes, we have been re-exploring distance determination through the
phase-lag method for a sample of stars, in order to refine the
poorly-constrained distances of some and infer the currently unknown distances
of others. We present here an update of this project.Comment: 4 pages, 1 figure, 2 tables, to appear in the Proceedings of the IAU
Symposium No. 336: Astrophysical Masers: Unlocking the Mysteries of the
Univers
Multi-wavelength Radio Continuum Emission Studies of Dust-free Red Giants
Multi-wavelength centimeter continuum observations of non-dusty,
non-pulsating K spectral-type red giants directly sample their chromospheres
and wind acceleration zones. Such stars are feeble emitters at these
wavelengths however, and previous observations have provided only a small
number of modest S/N measurements slowly accumulated over three decades. We
present multi-wavelength Karl G. Jansky Very Large Array thermal continuum
observations of the wind acceleration zones of two dust-free red giants,
Arcturus (Alpha Boo: K2 III) and Aldebaran (Alpha Tau: K5 III). Importantly,
most of our observations of each star were carried out over just a few days, so
that we obtained a snapshot of the different stellar atmospheric layers sampled
at different wavelengths, independent of any long-term variability. We report
the first detections at several wavelengths for each star including a detection
at 10 cm (3.0 GHz: S band) for both stars and a 20 cm (1.5 GHz: L band)
detection for Alpha Boo. This is the first time single luminosity class III red
giants have been detected at these continuum wavelengths. Our long-wavelength
data sample the outer layers of Alpha Boo's atmosphere where its wind velocity
is approaching its terminal value and the ionization balance is becoming
frozen-in. For Alpha Tau, however, our long-wavelength data are still sampling
its inner atmosphere, where the wind is still accelerating probably due to its
lower mass-loss rate. We compare our data with published semi-empirical models
based on ultraviolet data, and the marked deviations highlight the need for new
atmospheric models to be developed. Spectral indices are used to discuss the
possible properties of the stellar atmospheres, and we find evidence for a
rapidly cooling wind in the case of Alpha Boo. Finally, we develop a simple
analytical wind model for Alpha Boo based on our new long-wavelength flux
measurements
Temporal Evolution of the Size and Temperature of Betelgeuse's Extended Atmosphere
We use the Very Large Array (VLA) in the A configuration with the Pie Town
(PT) Very Long Baseline Array (VLBA) antenna to spatially resolve the extended
atmosphere of Betelgeuse over multiple epochs at 0.7, 1.3, 2.0, 3.5, and 6.1
cm. The extended atmosphere deviates from circular symmetry at all wavelengths
while at some epochs we find possible evidence for small pockets of gas
significantly cooler than the mean global temperature. We find no evidence for
the recently reported e-MERLIN radio hotspots in any of our multi-epoch VLA/PT
data, despite having sufficient spatial resolution and sensitivity at short
wavelengths, and conclude that these radio hotspots are most likely
interferometric artefacts. The mean gas temperature of the extended atmosphere
has a typical value of 3000 K at 2 and decreases to 1800 K at 6
, in broad agreement with the findings of the single epoch study
from Lim et al. (1998). The overall temperature profile of the extended
atmosphere between can be
described by a power law of the form ,
with temporal variability of a few 100 K evident at some epochs. Finally, we
present over 12 years of V band photometry, part of which overlaps our
multi-epoch radio data. We find a correlation between the fractional flux
density variability at V band with most radio wavelengths. This correlation is
likely due to shock waves induced by stellar pulsations, which heat the inner
atmosphere and ionize the more extended atmosphere through radiative means.
Stellar pulsations may play an important role in exciting Betelgeuse's extended
atmosphere
Characterising the high-mass star forming region IRAS 18144-1723 through methanol maser observations
High-z radio starbursts host obscured X-ray AGN
We use Virtual Observatory methods to investigate the association between
radio and X-ray emission at high redshifts. Fifty-five of the 92 HDF(N) sources
resolved by combining
MERLIN+VLA data were detected by Chandra, of which 18 are hard enough and
bright enough to be obscured AGN. The high-z population of microJy radio
sources is dominated by starbursts an order of magnitude more active and more
extended than any found at z<1 and at least a quarter of these simultaneously
host highly X-ray-luminous obscured AGN.Comment: 4 pages, 2 figures, To appear in the proceedings of 'At the Edge of
the Universe' (9-13 October 2006, Sintra, Portugal
OH maser towards IRAS 06056+2131: polarization parameters and evolution status
We present high angular resolution observations of OH maser emission towards
the high-mass star forming region IRAS 06056+2131. The observations were
carried out using the UK radio interferometer array, Multi-Element Radio Linked
Interferometer Network (MERLIN) in the OH main lines at 1665- and 1667-MHz, in
addition to the OH satellite line at 1720-MHz. The results of this study
revealed emission in the 1665 MHz line with an estimated total intensity of
Jy. We did not detect any emission from the 1667-MHz and 1720-MHz
lines.
The full polarization mode of MERLIN enables us to investigate the magnetic
field in the OH maser region. Our results show that IRAS 06056+2131 is a highly
circularly polarized source. In this transition, a Zeeman pair is identified
from which a magnetic strength of mG is inferred. The orientation
of the linear polarization vectors suggests that the magnetic field lines at
the location of the OH maser emission \textbf{might be} in agreement with the
orientation of the outflow thought to be associated with this source. The star
forming evolutionary status of the embedded proto-stellar object is discussed.Comment: 10 pages, 5 figure
Circumstellar CO in metal-poor stellar winds: the highly irradiated globular cluster star 47 Tucanae V3
We report the first detection of circumstellar CO in a globular cluster.
Observations with ALMA have detected the CO J=3-2 and SiO v=1 J=8-7 transitions
at 345 and 344 GHz, respectively, around V3 in 47 Tucanae (NGC 104; [Fe/H] =
-0.72 dex), a star on the asymptotic giant branch. The CO line is detected at 7
sigma at a rest velocity v_LSR = -40.6 km/s and expansion velocity of 3.2 +/-
~0.4 km/s. The brighter, asymmetric SiO line may indicate a circumstellar
maser. The stellar wind is slow compared to similar Galactic stars, but the
dust opacity remains similar to Galactic comparisons. We suggest that the
mass-loss rate is set by the levitation of material into the circumstellar
environment by pulsations, but that the terminal wind-expansion velocity is
determined by radiation pressure on the dust: a pulsation-enhanced dust-driven
wind. We suggest the metal-poor nature of the star decreases the grain size,
slowing the wind and increasing its density and opacity. Metallic alloys at
high altitudes above the photosphere could also provide an opacity increase.
The CO line is weaker than expected from Galactic AGB stars, but its strength
confirms a model that includes CO dissociation by the strong interstellar
radiation field present inside globular clusters.Comment: 5 pages, accepted MNRAS Letter
Phosphine on Venus Cannot be Explained by Conventional Processes
The recent candidate detection of ~1 ppb of phosphine in the middle
atmosphere of Venus is so unexpected that it requires an exhaustive search for
explanations of its origin. Phosphorus-containing species have not been
modelled for Venus' atmosphere before and our work represents the first attempt
to model phosphorus species in the Venusian atmosphere. We thoroughly explore
the potential pathways of formation of phosphine in a Venusian environment,
including in the planet's atmosphere, cloud and haze layers, surface, and
subsurface. We investigate gas reactions, geochemical reactions,
photochemistry, and other non-equilibrium processes. None of these potential
phosphine production pathways are sufficient to explain the presence of ppb
phosphine levels on Venus. If PH3's presence in Venus' atmosphere is confirmed,
it therefore is highly likely to be the result of a process not previously
considered plausible for Venusian conditions. The process could be unknown
geochemistry, photochemistry, or even aerial microbial life, given that on
Earth phosphine is exclusively associated with anthropogenic and biological
sources. The detection of phosphine adds to the complexity of chemical
processes in the Venusian environment and motivates in situ follow up sampling
missions to Venus. Our analysis provides a template for investigation of
phosphine as a biosignature on other worlds.Comment: v2 is in press in Astrobiology, Special Collection: Venus; v2 also
expands on the potential of phosphides from the deep mantle volcanism as a
source of PH3 (as suggested by Truong and Lunine 2021:
https://www.pnas.org/content/118/29/e2021689118) and shows the volcanic
source of PH3 to be unlikel
Venusian phosphine:a 'Wow!' signal in chemistry?
The potential detection of ppb levels phosphine (PH3) in the clouds of Venus
through millimeter-wavelength astronomical observations is extremely surprising
as PH3 is an unexpected component of an oxidized environment of Venus. A
thorough analysis of potential sources suggests that no known process in the
consensus model of Venus' atmosphere or geology could produce PH3 at anywhere
near the observed abundance. Therefore, if the presence of PH3 in Venus'
atmosphere is confirmed, it is highly likely to be the result of a process not
previously considered plausible for Venusian conditions. The source of
atmospheric PH3 could be unknown geo- or photochemistry, which would imply that
the consensus on Venus' chemistry is significantly incomplete. An even more
extreme possibility is that strictly aerial microbial biosphere produces PH3.
This paper summarizes the Venusian PH3 discovery and the scientific debate that
arose since the original candidate detection one year ago.Comment: A short overview of the Venusian PH3 discovery and the scientific
debate that arose since the original candidate detection in September 2020.
Additional discussion of possible non-canonical sources of PH3 on Venus is
also included. arXiv admin note: text overlap with arXiv:2009.0649
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