671 research outputs found
Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA
Planets form in protoplanetary disks and inherit their chemical compositions.
It is thus crucial to map the distribution and investigate the formation of
simple organics, such as formaldehyde and methanol, in protoplanetary disks. We
analyze ALMA observations of the nearby disk-jet system around the T Tauri star
DG Tau in the o-HCO and CHOH E,
A transitions at an unprecedented resolution of ,
i.e., au at a distance of 121 pc. The HCO emission originates from
a rotating ring extending from au with a peak at au, i.e., at
the edge of the 1.3mm dust continuum. CHOH emission is not detected down to
an r.m.s. of 3 mJy/beam in the 0.162 km/s channel. Assuming an ortho-to-para
ratio of 1.8-2.8 the ring- and disk-height-averaged HCO column density is
cm, while that of CHOH is
cm. In the inner au no o-HCO emission
is detected with an upper limit on its beam-averaged column density of
cm. The HCO ring in the disk of DG Tau is
located beyond the CO iceline (R au). This suggests that the
HCO abundance is enhanced in the outer disk due to formation on grain
surfaces by the hydrogenation of CO ice. The emission peak at the edge of the
mm dust continuum may be due to enhanced desorption of HCO in the gas phase
caused by increased UV penetration and/or temperature inversion. The
CHOH/HCO abundance ratio is , in agreement with disk chemistry
models. The inner edge of the HCO ring coincides with the radius where the
polarization of the dust continuum changes orientation, hinting at a tight link
between the HCO chemistry and the dust properties in the outer disk and at
the possible presence of substructures in the dust distribution.Comment: 8 pages, 6 figures, accepted for publication on A&A Letter
A Deep Look at the T-Type Brown Dwarf Binary Indi Bab with Chandra and ATCA
We present deep observations of the nearby T-type brown dwarf binary
Indi Bab in radio with the Australia Telescope Compact Array and in
X-rays with the Chandra X-ray Observatory. Despite long integration times, the
binary (composed of T1 and T6 dwarfs) was not detected in either wavelength
regime. We reached upper limits of and erg/s/Hz for the radio luminosity at 4.8 GHz and 8.64 GHz,
respectively; in the X-rays, the upper limit in the keV band was erg/s. We discuss the above results in the framework of
magnetic activity in ultracool, low-mass dwarfs.Comment: Accepted for publication in the Astrophysical Journal Letter
Flux growth and liquid phase epitaxy of undoped and Mn6+-doped sulfates, tungstates, and molybdates
The Mn6+ ion is a promising activator ion for tunable and short-pulse laser materials because of its broadband luminescence in the spectral region 850-1600 nm and its simple 3d1 electronic configuration, which excludes an occurrence of undesirable exited-state absorption into higher 3d levels. However, hexavalent manganese can be stabilized only in the tetrahedral oxo-coordination and easily reduces to Mn5+ or Mn4+ at temperatures above 600°C. Recently, flux [1] and liquid-phase epitaxy (LPE) [2] growth of Mn6+-doped sulfates has been reported, while except for BaMoO4:Mn6+ [3] investigations on the mechanically more stable alkaline-earth-metal molybdates and tungstates as possible host materials for efficient Mn6+ incorporation have as yet not been reported.\ud
We investigated the growth conditions of undoped and Mn6+-doped MAO4, with M = Ca, Sr, Ba and A = S, Mo, W, from the ternary NaCl-KCl-CsCl solvent at temperatures 480-600°C. The growth rates increase in the series tungstates < molybdates < sulfates and depending on the cation, in the series Ca < Sr < Ba. The dopant ion Mn6+ can be easily incorporated into BaSO4, less well into BaMoO4 and BaWO4, whereas for Ca- and Sr-containing tungstates and molybdates no significant doping was found, independent on the concentration of Mn6+ in the liquid solution. Moreover, reduction of the Mn6+ ion cannot be avoided, even at the presence of oxidizing additives such as K2CO3 or NaOH.\ud
LPE was employed for growing Mn6+-doped layers of BaAO4 compounds. Growth velocities of 3-5 µm/h in the temperature interval from 490-540°C from chloridic solution, containing 0.3-1mol% of K2MnO4 with respect to the solute, delivered dark-pink BaSO4 and slightly green BaMoO4 and BaWO4 layers up to 200 µm in thickness. With respect to high Mn6+ doping levels, BaSO4 is the most suitable host material and its further investigation under different initial concentrations of manganese is currently underway.\ud
\ud
[1] T.C. Brunold, H.U. Güdel, Inorg. Chem. 36, 1946 (1997).\ud
[2] D. Ehrentraut, M. Pollnau, Appl. Phys. B 75, 59 (2002).\ud
[3] T.C. Brunold, H.U. Güdel, Chem. Phys. Lett. 249, 77 (1996)
The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star
Aims. We aim to describe the pre-main sequence and main-sequence evolution of
X-ray and extreme-ultaviolet radiation of a solar mass star based on its
rotational evolution starting with a realistic range of initial rotation rates.
Methods. We derive evolutionary tracks of X-ray radiation based on a
rotational evolution model for solar mass stars and the rotation-activity
relation. We compare these tracks to X-ray luminosity distributions of stars in
clusters with different ages.
Results. We find agreement between the evolutionary tracks derived from
rotation and the X-ray luminosity distributions from observations. Depending on
the initial rotation rate, a star might remain at the X-ray saturation level
for very different time periods, approximately from 10 Myr to 300 Myr for slow
and fast rotators, respectively.
Conclusions. Rotational evolution with a spread of initial conditions leads
to a particularly wide distribution of possible X-ray luminosities in the age
range of 20 to 500 Myrs, before rotational convergence and therefore X-ray
luminosity convergence sets in. This age range is crucial for the evolution of
young planetary atmospheres and may thus lead to very different planetary
evolution histories.Comment: 4 pages, 4 figures, accepted for publication in A&
Meter- to Millimeter Emission from Cool Stellar Systems : Latest Results, Synergies Across the Spectrum, and Outlook for the Next Decade
Splinter session summary, to appear in the proceedings of the 20th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (ed. S. J. Wolk)Radio observations of cool stellar systems provide unique information on their magnetic fields, high-energy processes, and chemistry. Buoyed by powerful new instruments (e.g. ALMA, JVLA, LOFAR), advances in related fields (e.g., the Gaia astrometric revolution), and above all a renewed interest in the relevant stellar astrophysics, stellar radio astronomy is experiencing a renaissance. In this splinter session, participants took stock of the present state of stellar radio astronomy to chart a course for the field's future
The JCMT Transient Survey: An Extraordinary Submillimeter Flare in the T Tauri Binary System JW 566
© 2019 The American Astronomical Society. All rights reserved.The binary T Tauri system JW 566 in the Orion Molecular Cloud underwent an energetic, short-lived flare observed at submillimetre wavelengths by the SCUBA-2 instrument on 26 November 2016 (UT). The emission faded by nearly 50% during the 31 minute integration. The simultaneous source fluxes averaged over the observation are 500 +/- 107 mJy/beam at 450 microns and 466 +/- 47 mJy/beam at 850 microns. The 850 micron flux corresponds to a radio luminosity of erg/s/Hz, approximately one order of magnitude brighter (in terms of ) than that of a flare of the young star GMR-A, detected in Orion in 2003 at 3mm. The event may be the most luminous known flare associated with a young stellar object and is also the first coronal flare discovered at sub-mm wavelengths. The spectral index between 450 microns and 850 microns of is broadly consistent with non-thermal emission. The brightness temperature was in excess of K. We interpret this event to be a magnetic reconnection that energised charged particles to emit gyrosynchrotron/synchrotron radiation.Peer reviewedFinal Published versio
Heating efficiency in hydrogen-dominated upper atmospheres
Context. The heating efficiency is defined as the ratio of the net local
gas-heating rate to the rate of stellar radiative energy absorption. It plays
an important role in thermal-escape processes from the upper atmospheres of
planets that are exposed to stellar soft X-rays and extreme ultraviolet
radiation (XUV). Aims. We model the thermal-escape-related heating efficiency
of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the
extrasolar gas giant HD 209458b. The model result is then compared with
previous thermal-hydrogen-escape studies which assumed heating efficiency
values between 10-100%. Methods. The photolytic and electron impact processes
in the thermosphere were studied by solving the kinetic Boltzmann equation and
applying a Direct Simulation Monte Carlo model. We calculated the energy
deposition rates of the stellar XUV flux and that of the accompanying primary
photoelectrons that are caused by electron impact processes in the H2 to H
transition region in the upper atmosphere. Results. The heating by XUV
radiation of hydrogen-dominated upper atmospheres does not reach higher than
20% above the main thermosphere altitude, if the participation of photoelectron
impact processes is included. Conclusions. Hydrogen-escape studies from
exoplanets that assume heating efficiency values that are >= 20 % probably
overestimate the thermal escape or mass-loss rates, while those who assumed
values that are < 20% probably produce more realistic atmospheric-escape rates.Comment: 7 pages, 4 figures, accepted to A&
Phase connecting multi-epoch radio data for the ultracool dwarf TVLM 513-46546
Context. Radio data obtained for the ultracool dwarf TVLM 513-46546 has indicated a rotation period of ≈1.96 h via regular radio pulses, but how stable is this period. This has major implications regarding the stability of the magnetic field structures responsible for the radio emission from the ultracool dwarf.
Aims. The aim of the present work is to investigate the stability of this rotation period using two datasets taken ≈40 days apart, some 12 months after the first report of periodical pulses in the radio data.
Methods. Here we use a Bayesian analysis method which is a statistical procedure that endeavours to estimate the parameters of an underlying model probability distribution based on the observed data.
Results. Periodical pulses are detected in datasets taken in April and June 2007, with the pulses being confined to a narrow range in the rotation period. This is in contradiction to a previous report of only aperiodic activity in the April 2007 dataset, while in fact both datasets have a periodic signal with a false alarm probability ≪ 10^-12. These two datasets are then used to derive a more accurate period (previously determined to be 1.96 h) of 1.96733 ± 0.00002 h.
Conclusions. The similarly in the burst structure in datasets taken several weeks apart point towards the stability of an electric field structure which is somehow generated and sustained within the magnetosphere of the ultracool dwarf. The derived period of 1.96733 h is consistent with the period derived via radio and optical data taken some 12 months prior to the present observations and implies the near phase constancy of the pulsed emission. This suggest the presence of stable large-scale magnetic fields on timescales of more than 1 year. The characteristics of the pulses suggest that they are produced by the electron cyclotron maser (ECM) instability
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