101 research outputs found
The Radio Spectrum of TVLM513-46546: Constraints on the Coronal Properties of a Late M Dwarf
We explore the radio emission from the M9 dwarf, TVLM513-46546, at multiple
radio frequencies, determining the flux spectrum of persistent radio emission,
as well as constraining the levels of circular polarization. Detections at both
3.6 and 6 cm provide spectral index measurement (where S) of . A detection at 20 cm suggests that the
spectral peak is between 1.4 and 5 GHz. The most stringent upper limits on
circular polarization are at 3.6 and 6 cm, with 15%. These
characteristics agree well with those of typical parameters for early to mid M
dwarfs, confirming that magnetic activity is present at levels comparable with
those extrapolated from earlier M dwarfs. We apply analytic models to
investigate the coronal properties under simple assumptions of dipole magnetic
field geometry and radially varying nonthermal electron density distributions.
Requiring the spectrum to be optically thin at frequencies higher than 5 GHz
and reproducing the observed 3.6 cm fluxes constrains the magnetic field at the
base to be less than about 500 G. There is no statistically significant
periodicity in the 3.6 cm light curve, but it is consistent with low-level
variability.Comment: 11 pages, 2 figures Accepted for publication in the Astrophysical
Journa
Constraining the Radio Emission of TRAPPIST-1
TRAPPIST-1 is an ultracool dwarf (UCD) with a system of 7 terrestrial
planets, at least three of which orbit in the habitable zone. The radio
emission of such low-mass stars is poorly understood; few UCDs have been
detected at radio frequencies at all, and the likelihood of detection is only
loosely correlated with stellar properties. Relative to other low-mass stars,
UCDs with slow rotation such as TRAPPIST-1 tend to be radio dim, whereas
rapidly rotating UCDs tend to have strong radio emission - although this is not
always the case. We present radio observations of TRAPPIST-1 using ALMA at 97.5
GHz and the VLA at 44 GHz. TRAPPIST-1 was not detected at either frequency and
we place upper flux limits of 10.6 and 16.2 Jy, respectively.
We use our results to constrain the magnetic properties and possible outgoing
high energy particle radiation from the star. The presence of radio emission
from UCDs is indicative of a stellar environment that could pose a threat to
life on surrounding planets. Gyrosynchrotron emission, discernible at
frequencies between 20 and 100 GHz, is one of the only processes that can be
used to infer the presence of high energy particles released during magnetic
reconnection events. Since M dwarfs are frequent hosts of terrestrial planets,
characterizing their stellar emission is a crucial part of assessing
habitability. Exposure to outgoing high energy particle radiation - traceable
by radio flux - can erode planetary atmospheres. While our results do not imply
that the TRAPPIST-1 planets are suitable for life, we find no evidence that
they are overtly unsuitable due to proton fluxes.Comment: 9 pages, 3 figures, accepted to Ap
X-ray Flares of EV Lac: Statistics, Spectra, Diagnostics
We study the spectral and temporal behavior of X-ray flares from the active
M-dwarf EV Lac in 200 ks of exposure with the Chandra/HETGS. We derive flare
parameters by fitting an empirical function which characterizes the amplitude,
shape, and scale. The flares range from very short (<1 ks) to long (10 ks)
duration events with a range of shapes and amplitudes for all durations. We
extract spectra for composite flares to study their mean evolution and to
compare flares of different lengths. Evolution of spectral features in the
density-temperature plane shows probable sustained heating. The short flares
are significantly hotter than the longer flares. We determined an upper limit
to the Fe K fluorescent flux, the best fit value being close to what is
expected for compact loops.Comment: 9 pages; 9 figures; latex/emulateapj style; Submitted to The
Astrophysical Journa
A Very Bright, Very Hot, and Very Long Flaring Event from the M Dwarf Binary System DG CVn
On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DG CVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3–100 keV bandpass by either a single very high-temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at T ( )X( ) of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be >10(20) cm(2), and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T ∼ 10(4) K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3–10 keV bandpass of 4 × 10(35) and 9 × 10(35) erg, and optical flare energies at E ( )V( ) of 2.8 × 10(34) and 5.2 × 10(34) erg, respectively. The results presented here should be integrated into updated modeling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres
- …