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Lanz, Oswald; Lehnartz,; Ulrici,; Keilhack, Heinz; Fischer, A. W.; Curschmann, Hans; Schulten,; Mader,

doi:10.1007/bf01775341

Cited by 5 publications

(5 citation statements)

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“…This may be related to the fact that there are no direct observational measurements for mass loss rates in main sequence A-type stars. The upper limits derived from the observations (Brown et al 1990;Lanz & Catala 1992) are 2-3 orders of magnitude above very few estimations (Bruhweiler et al 1991;Bertin et al 1995) we have for such stars.…”

Section: Interaction With the Windsupporting

confidence: 38%

Search for exoplanets and brown dwarfs with VLBI

Gawroński

Goździewski

2016

Mon. Not. R. Astron. Soc.

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The main aim of this work is to estimate possible radio GHz emission of extrasolar planets and brown dwarfs and to check if such radiation can be detected by Very Large Baseline Interferometers (VLBI). In the estimation we assume that the emission may originate in processes similar to those observed in the Jupiter system. The frequency of the radio emission that is produced in this system depends mostly on the magnetic field strength. Jupiter's magnetic field (∼ 9 G on average) allows for radiation from kHz frequencies up to 40 MHz. This is well below the frequency range of VLBI. However, it was demonstrated that the magnetic field strength in massive and young object may be up to two orders of magnitude higher than for Jupiter, which is especially relevant for planets around short-lived A type stars. This should extend the range of the emission up to GHz frequencies. We calculated expected flux densities of radio emission for a variety of hypothetical young planetary systems. We analysed two different emission scenarios, and found that the radiation induced by moons (process similar to Jupiter-Io interactions) appears to be less efficient than the emission generated by a stellar wind on a planetary magnetosphere. We also estimated hypothetical emission of planets and brown dwarfs located around relatively young and massive main sequence A-type stars. Our results show that the emission produced by stellar winds could be detected by currently operating VLBI networks.

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Section: Interaction With the Windsupporting

confidence: 38%

Search for exoplanets and brown dwarfs with VLBI

Gawroński

Goździewski

2016

Mon. Not. R. Astron. Soc.

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“…First, A-star winds are feeble. An upper limit on the mass loss from Vega, for example, iṡ M 4 × 10 −10 M yr −1 (Lanz and Catala 1992), which corresponds to mean electron densities n e 10 −6 cm −3 at 2 pc, if the wind velocity is 300 km s −1 . By contrast, the IDV phenomenon requires values ∼ 10 cm −3 (Tuntsov, Bignall and Walker 2013); the stellar wind is not a natural source of such dense plasma.…”

Section: Nature Of the Plasma Filamentsmentioning

confidence: 99%

Extreme Radio-wave Scattering Associated with Hot Stars

Walker

Tuntsov

Bignall

et al. 2017

ApJ

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We use data on extreme radio scintillation to demonstrate that this phenomenon is associated with hot stars in the solar neighborhood. The ionized gas responsible for the scattering is found at distances up to 1.75 pc from the host star, and on average must comprise ∼10 5 distinct structures per star. We detect azimuthal velocities of the plasma, relative to the host star, up to 9.7 km s 1 -, consistent with warm gas expanding at the sound speed. The circumstellar plasma structures that we infer are similar in several respects to the cometary knots seen in the Helix and in other planetary nebulae. There the ionized gas appears as a skin around tiny molecular clumps. Our analysis suggests that molecular clumps are ubiquitous circumstellar features, unrelated to the evolutionary state of the star. The total mass in such clumps is comparable to the stellar mass.

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“…For these early-type stars, stellar winds may also play a crucial role in disk dispersal. Using the relationship for the stellar wind dust blowout radius from Plavchan et al (2005) and the wind mass loss rate upper limit (2 × 10 −10 M ⊙ yr −1 ) for main sequence A-type stars from Lanz & Catala (1992), we estimate the wind dust blowout radii to be an order of magnitude less than those from radiation pressure alone. While the mass-loss rates for young stars may be higher, we adopt the latter values to constrain the minimum dust grain sizes around these early-type mid-infrared excess stars.…”

Section: Circumstellar Disks Associated With Early-type Upper Scorpiumentioning

confidence: 99%

SPITZERSPECTROSCOPY OF CIRCUMSTELLAR DISKS IN THE 5 Myr OLD UPPER SCORPIUS OB ASSOCIATION

Dahm

Carpenter

2009

The Astronomical Journal

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We present mid-infrared spectra between 5.2 and 38 µm for 26 disk-bearing members of the ∼5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ∼8 µm. In contrast, 10 and 20 µm silicate features or PAH emission are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral type dependent, with the most prominent features being associated with K5-M2 type stars. By fitting the spectral energy distributions (SED) of a representative sample of lowmass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ∼0.2 to 1.2 AU. Complementary high resolution (R ∼ 33, 000) optical (λλ4800-9200) spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only 7 (all late-type) exhibit definitive signatures of accretion. Mass accretion rates (Ṁ ) for these stars were estimated to range from 10 −11 to 10 −8.9 M ⊙ yr −1 . Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near and mid-infrared excess emission and an order of magnitude lower mass accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time.

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Cited by 5 publications

References 0 publications

Search for exoplanets and brown dwarfs with VLBI

Search for exoplanets and brown dwarfs with VLBI

Extreme Radio-wave Scattering Associated with Hot Stars

SPITZERSPECTROSCOPY OF CIRCUMSTELLAR DISKS IN THE 5 Myr OLD UPPER SCORPIUS OB ASSOCIATION

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