1,651 research outputs found
Detection of thermal radio emission from a single coronal giant
We report the detection of thermal continuum radio emission from the K0 III
coronal giant Pollux ( Gem) with the Karl G. Jansky Very Large Array
(VLA). The star was detected at 21 and 9 GHz with flux density values of
and Jy, respectively. We also place a
upper limit of Jy for the flux density at 3
GHz. We find the stellar disk-averaged brightness temperatures to be
approximately 9500, 15000, and K, at 21, 9, and 3 GHz, respectively,
which are consistent with the values of the quiet Sun. The emission is most
likely dominated by optically thick thermal emission from an upper chromosphere
at 21 and 9 GHz. We discuss other possible additional sources of emission at
all frequencies and show that there may also be a small contribution from
gyroresonance emission above active regions, coronal free-free emission and
free-free emission from an optically thin stellar wind, particularly at the
lower frequencies. We constrain the maximum mass-loss rate from Pollux to be
less than yr (assuming a wind terminal
velocity of 215 km s), which is about an order of magnitude smaller than
previous constraints for coronal giants and is in agreement with existing
predictions for the mass-loss rate of Pollux. These are the first detections of
thermal radio emission from a single (i.e., non-binary) coronal giant and
demonstrate that low activity coronal giants like Pollux have atmospheres at
radio frequencies akin to the quiet Sun
Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars
We report on an ultraviolet spectroscopic survey of red giants observed by
the Hubble Space Telescope, focusing on spectra of the Mg II h & k lines near
2800 A in order to study stellar chromospheric emission, winds, and
astrospheric absorption. We focus on spectral types between K2 III and M5 III,
a spectral type range with stars that are noncoronal, but possessing strong,
chromospheric winds. We find a very tight relation between Mg II surface flux
and photospheric temperature, supporting the notion that all K2-M5 III stars
are emitting at a basal flux level. Wind velocities (V_w) are generally found
to decrease with spectral type, with V_w decreasing from ~40 km/s at K2 III to
~20 km/s at M5 III. We find two new detections of astrospheric absorption, for
Sigma Pup (K5 III) and Gamma Eri (M1 III). This absorption signature had
previously only been detected for Alpha Tau (K5 III). For the three
astrospheric detections the temperature of the wind after the termination shock
correlates with V_w, but is lower than predicted by the Rankine-Hugoniot shock
jump conditions, consistent with the idea that red giant termination shocks are
radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic
simulation of the Gamma Eri astrosphere is provided to explore this further.Comment: 16 pages, 8 figures, to appear in The Astrophysical Journa
Evolved Late-Type Star FUV Spectra: Mass Loss and Fluorescence
This proposal was for a detailed analysis of the far ultraviolet (FUV) photoionizing radiation that provides crucial input physics for mass loss studies, e.g., observations of the flux below 10448, allow us to constrain the Ca II/Ca III balance and make significant progress beyond previous optical studies on stellar mass loss and circumstellar photochemistry. Our targets selection provided good spectral-type coverage required to help unravel the Ca II/Ca III balance as the mass-loss rates increase by over three orders of magnitude from K5 III to M5 III. We also explored the relationship between the FUV radiation field and other UV diagnostics to allow us to empirically estimate the FUV radiation field for the vast majority of stars which are too faint to be observed with FUSE, and to improve upon their uncertain mass-loss rates
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
CARMA CO(J = 2 - 1) Observations of the Circumstellar Envelope of Betelgeuse
We report radio interferometric observations of the 12C16O 1.3 mm J = 2-1
emission line in the circumstellar envelope of the M supergiant Alpha Ori and
have detected and separated both the S1 and S2 flow components for the first
time. Observations were made with the Combined Array for Research in
Millimeter-wave Astronomy (CARMA) interferometer in the C, D, and E antenna
configurations. We obtain good u-v coverage (5-280 klambda) by combining data
from all three configurations allowing us to trace spatial scales as small as
0.9\arcsec over a 32\arcsec field of view. The high spectral and spatial
resolution C configuration line profile shows that the inner S1 flow has
slightly asymmetric outflow velocities ranging from -9.0 km s-1 to +10.6 km s-1
with respect to the stellar rest frame. We find little evidence for the outer
S2 flow in this configuration because the majority of this emission has been
spatially-filtered (resolved out) by the array. We also report a SOFIA-GREAT
CO(J= 12-11) emission line profile which we associate with this inner higher
excitation S1 flow. The outer S2 flow appears in the D and E configuration maps
and its outflow velocity is found to be in good agreement with high resolution
optical spectroscopy of K I obtained at the McDonald Observatory. We image both
S1 and S2 in the multi-configuration maps and see a gradual change in the
angular size of the emission in the high absolute velocity maps. We assign an
outer radius of 4\arcsec to S1 and propose that S2 extends beyond CARMA's field
of view (32\arcsec at 1.3 mm) out to a radius of 17\arcsec which is larger than
recent single-dish observations have indicated. When azimuthally averaged, the
intensity fall-off for both flows is found to be proportional to R^{-1}, where
R is the projected radius, indicating optically thin winds with \rho \propto
R^{-2}.Comment: 11 pages, 8 figures To be published in the Astronomical Journal
(Received 2012 February 10; accepted 2012 May 25
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
Hubble Space Telescopeconstraints on the Winds and Astrospheres of Red Giant Stars
We report on an ultraviolet spectroscopic survey of red giants observed by the Hubble Space Telescope, focusing on spectra of the Mg ii h and k lines near 2800 Å in order to study stellar chromospheric emission, winds, and astrospheric absorption. We focus on spectral types between K2 III and M5 III, a spectral type range with stars that are noncoronal, but possessing strong, chromospheric winds. We find a very tight relation between Mg ii surface flux and photospheric temperature, supporting the notion that all K2-M5 III stars are emitting at a basal flux level. Wind velocities (V w ) are generally found to decrease with spectral type, with V w decreasing from ~40 km s−1 at K2 III to ~20 km s−1 at M5 III. We find two new detections of astrospheric absorption, for σ Pup (K5 III) and γ Eri (M1 III). This absorption signature had previously only been detected for α Tau (K5 III). For the three astrospheric detections, the temperature of the wind after the termination shock (TS) correlates with V w , but is lower than predicted by the Rankine–Hugoniot shock jump conditions, consistent with the idea that red giant TSs are radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic simulation of the γ Eri astrosphere is provided to explore this further
The Photospheric Temperatures of Betelgeuse during the Great Dimming of 2019/2020: No New Dust Required
The processes that shape the extended atmospheres of red supergiants (RSGs),
heat their chromospheres, create molecular reservoirs, drive mass loss, and
create dust remain poorly understood. Betelgeuse's V-band "Great Dimming" event
of 2019 September /2020 February and its subsequent rapid brightening provides
a rare opportunity to study these phenomena. Two different explanations have
emerged to explain the dimming; new dust appeared in our line of sight
attenuating the photospheric light, or a large portion of the photosphere had
cooled. Here we present five years of Wing three-filter (A, B, and C band) TiO
and near-IR photometry obtained at the Wasatonic Observatory. These reveal that
parts of the photosphere had a mean effective temperature )
significantly lower than that found by (Levesque & Massey 2020). Synthetic
photometry from MARCS -model photospheres and spectra reveal that the V band,
TiO index, and C-band photometry, and previously reported 4000-6800 Angstrom
spectra can be quantitatively reproduced if there are multiple photospheric
components, as hinted at by VLT-SPHERE images (Montarges et al. 2020). If the
cooler component has K cooler than 3650 K, then no
new dust is required to explain the available empirical constraints. A
coincidence of the dominant short- ( day) and long-period (
yr) V-band variations occurred near the time of deep minimum (Guinan et al.
2019). This is in tandem with the strong correlation of V mag and photospheric
radial velocities, recently reported by Dupree et al. (2020b). These suggest
that the cooling of a large fraction of the visible star has a dynamic origin
related to the photospheric motions, perhaps arising from pulsation or
large-scale convective motions.Comment: Accepted ApJ - 19 pages, 5 figure
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