20 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
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
A search for radio emission from exoplanets around evolved stars
The majority of searches for radio emission from exoplanets have to date
focused on short period planets, i.e., the so-called hot Jupiter type planets.
However, these planets are likely to be tidally locked to their host stars and
may not generate sufficiently strong magnetic fields to emit electron cyclotron
maser emission at the low frequencies used in observations (typically >150
MHz). In comparison, the large mass-loss rates of evolved stars could enable
exoplanets at larger orbital distances to emit detectable radio emission. Here,
we first show that the large ionized mass-loss rates of certain evolved stars
relative to the solar value could make them detectable with the Low Frequency
Array (LOFAR) at 150 MHz ( = 2 m), provided they have surface magnetic
field strengths >50 G. We then report radio observations of three long period
(>1 au) planets that orbit the evolved stars Gem, Dra, and
UMi using LOFAR at 150 MHz. We do not detect radio emission from any
system but place tight 3 upper limits of 0.98, 0.87, and 0.57 mJy on
the flux density at 150 MHz for Gem, Dra, and UMi,
respectively. Despite our non-detections these stringent upper limits highlight
the potential of LOFAR as a tool to search for exoplanetary radio emission at
meter wavelengths.Comment: 9 pages, 3 figure
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
Recent Transits of the Super-Earth Exoplanet GJ 1214b
We report recent ground-based photometry of the transiting super-Earth
exoplanet GJ1214b at several wavelengths, including the infrared near 1.25
microns (J-band). We observed a J-band transit with the FLAMINGOS infrared
imager and the 2.1-meter telescope on Kitt Peak, and we observed several
optical transits using a 0.5-meter telescope on Kitt Peak and the 0.36-meter
Universidad de Monterrey Observatory telescope. Our high-precision J-band
observations exploit the brightness of the M-dwarf host star at this infrared
wavelength as compared to the optical, as well as being significantly less
affected by stellar activity and limb darkening. We fit the J-band transit to
obtain an independent determination of the planetary and stellar radii. Our
radius for the planet (2.61^+0.30_-0.11 Earth radii) is in excellent agreement
with the discovery value reported by Charbonneau et al. based on optical data.
We demonstrate that the planetary radius is insensitive to degeneracies in the
fitting process. We use all of our observations to improve the transit
ephemeris, finding P=1.5804043 +/- 0.0000005 days, and T0=2454964.94390 +/-
0.00006 BJD.Comment: Accepted for ApJ Letters, 7 pages, 3 Figures, 2 Table
Recommended from our members
The Wind Temperature and Mass-loss Rate of Arcturus (K1.5 III)
In this paper we aim to constrain the wind temperature, outflow and turbulent velocities, ionization state, and mass-loss rate of the single red giant Arcturus (α Boo K1.5 III) using high spectral resolution Hubble Space Telescope Space Telescope Imaging Spectrograph profiles of Si iii 1206.5 Å , O i 1304 Å and 1306 Å, C ii 1334 Å and 1335 Å, and Mg ii h 2802 Å. The use of the E140-H setting for α Boo allows the Si iii 1206.5 Å line to be cleanly extracted from the echelle format for the first time. The ratios of the wind optical depths of lines from different species constrain the temperature at the base of the wind to Twind ∼ 15,400 K. The mass-loss rate derived is 2.5 × 10−11 for Epoch 2018–2019, smaller than previous semiempirical estimates. These results can be reconciled with multiwavelength Very Large Array radio continuum fluxes for Epoch 2011–2012 by increasing the temperature to Twind ∼ 18,000 K, or increasing the mass-loss rate to 4.0 × 10−11 . Interpreting the wind acceleration and turbulence in terms of a steady WKB Alfvén wave–driven wind reveals that the wave energy damping length increases with increasing radius, opposite to the trend expected for ion-neutral damping of monochromatic waves, confirming a previous result by Kuin and Ahmad derived for ζ Aur binaries. This implies that a spectrum of waves is required in this framework with wave periods in the range of hours to days, consistent with the photospheric granulation timescale. Constraints on a radial magnetic field (B) at 1.2 R* are an upper limit of B ≤ 2 G from the implied wave heating, and B ≥ 0.3 G to avoid excessive wave amplitudes.
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The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA
Our current understanding of the chemistry and mass-loss processes in
solar-like stars at the end of their evolution depends critically on the
description of convection, pulsations and shocks in the extended stellar
atmosphere. Three-dimensional hydrodynamical stellar atmosphere models provide
observational predictions, but so far the resolution to constrain the complex
temperature and velocity structures seen in the models has been lacking. Here
we present submillimeter continuum and line observations that resolve the
atmosphere of the asymptotic giant branch star W Hya. We show that hot gas with
chromospheric characteristics exists around the star. Its filling factor is
shown to be small. The existence of such gas requires shocks with a cooling
time larger than commonly assumed. A shocked hot layer will be an important
ingredient in the models of stellar convection, pulsation and chemistry that
underlie our current understanding of the late stages of stellar evolution.Comment: 30 pages, 9 figures, including Supplementary information. Author
manuscript version before editorial/copyediting by Nature Astronomy. Journal
version available via http://rdcu.be/xUW
Imaging of Stellar Surfacess using radio facilities including ALMA
Until very recently, studies focusing on imaging stars at continuum radio wavelengths (here defined as submillimeter, millimeter, and centimeter wavelengths) has been scarce. These studies have mainly been carried out with the Very Large Array on a handful of evolved stars (i.e., Asymptotic Giant Branch and Red Supergiant stars) whereby their stellar disks have just about been spatially resolved. Some of these results however, have challenged our historical views on the nature of evolved star atmospheres. Now, the very long baselines of the Atacama Large Millimeter/submillimeter Array and the newly upgraded Karl G. Jansky Very Large Array provide a new opportunity to image these atmospheres at unprecedented spatial resolution and sensitivity across a much wider portion of the radio spectrum. In this talk I will first provide a history of stellar radio imaging and then discuss some recent exciting ALMA results. Finally I will present some brand new multi-wavelength ALMA and VLA results for the famous red supergiant Antares
Imaging of Stellar Surfacess using radio facilities including ALMA
Until very recently, studies focusing on imaging stars at continuum radio wavelengths (here defined as submillimeter, millimeter, and centimeter wavelengths) has been scarce. These studies have mainly been carried out with the Very Large Array on a handful of evolved stars (i.e., Asymptotic Giant Branch and Red Supergiant stars) whereby their stellar disks have just about been spatially resolved. Some of these results however, have challenged our historical views on the nature of evolved star atmospheres. Now, the very long baselines of the Atacama Large Millimeter/submillimeter Array and the newly upgraded Karl G. Jansky Very Large Array provide a new opportunity to image these atmospheres at unprecedented spatial resolution and sensitivity across a much wider portion of the radio spectrum. In this talk I will first provide a history of stellar radio imaging and then discuss some recent exciting ALMA results. Finally I will present some brand new multi-wavelength ALMA and VLA results for the famous red supergiant Antares
The Wind Acceleration Region of Betelgeuse: Resolved at Centimeter Wavelengths
We present multi-epoch spatially resolved radio continuum observations of Betelgeuse (alpha Ori) at various combinations of wavelengths between 0.7 and 6.1 cm. We used the Very Large Array in the A configuration with the Pie Town antenna to spatially resolve its atmosphere at 0.7, 1.3, 2.0, and 6.1 cm at all epochs. Our findings are similar to those of Lim et al. (1998) in that Betelgeuse's opaque atmosphere extends from 2 to 612(star) between 0.7 and 6.1 cm with temperatures decreasing from similar to 3000 to 1800 K, respectively. We find no evidence of radio hotspots at any epochs even though we have sufficient spatial resolution and sensitivity at 0.7 and 1.3 cm to detect the hotspots recently reported with e-MERLIN at 5.2 cm