Far‐Ultraviolet Continuum of G‐Type Stars: A Signature of the Temperature Minimum Region

Franchini, M.; Morossi, C.; Malagnini, M. L.

doi:10.1086/306400

Cited by 18 publications

(25 citation statements)

References 8 publications

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“…Each star presents a temperature-minimum region, a chromospheric rise followed by a plateau, and a steep temperature rise to the transition region. These confirm and expand the results of Franchini et al (1998) and Morossi et al (2003) who using a different wavelength range, the ultraviolet continua, also found that a chromospheric temperature rise was necessary. Pasquini (1992) studied observations of the Ca II K line for a sample of solar-type stars (G0−G5 in spectral type) and found that, as activity increases, first the intensity of the K 1 minimum increases, and for larger activity levels also the intensity of the K 2 maxima and of the central reversal K 3 change.…”

Section: Discussionsupporting

confidence: 89%

“…These stars are considered as "basal" stars due to their low chromospheric activity level. Franchini et al (1998) and Morossi et al (2003), using IUE and HST ultraviolet spectra, derived empirical temperature distributions in the atmosphere of G and K-type stars, paying particular attention to the temperature minimum region. They compared different atmospheric models and obtained the best agreement with observations with a temperature increasing outward of the temperature minimum zone.…”

Section: Introductionmentioning

confidence: 99%

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Chromospheric models of solar analogues with different activity levels

Vieytes

Mauas

Cincunegui

2005

A&A

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Abstract.We computed chromospheric models of the Sun as a star and of nine solar analogues. The atmospheric models were constructed to obtain the best possible match with the Ca II K and Hβ lines, including the asymmetry of the lines due to macroscopic velocity fields. The stars were chosen with 0.62 < B − V < 0.68 (the solar B − V = 0.65) and have a wide variety of magnetic activity levels, which allows us to study the differences in atmospheric structures induced by activity. For the less active stars we found that the changes with activity are in the region of the temperature minimum, while the most active stars show changes all along their atmospheric structures, mainly in the upper chromosphere. Regarding the macroscopic velocity fields, we can distinguish between the two groups. The most active group has a velocity field in the temperature-minimum region, and the other group in the chromospheric plateau. We also computed the net radiative losses for each model, and found that they depend linearly on the usual index of chromospheric activity, S CaII .

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Section: Discussionsupporting

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Chromospheric models of solar analogues with different activity levels

Vieytes

Mauas

Cincunegui

2005

A&A

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“…6. active chromosphere of class II objects (Houdebine et al 1996;Franchini et al 1998;Ingleby et al 2011b, see Fig. 14).…”

Section: Hα Hβ and Heimentioning

confidence: 99%

X-shooter spectroscopy of young stellar objects

Rigliaco

Natta

Testi

et al. 2012

A&A

177

291

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We present high-quality, medium-resolution X-shooter/VLT spectra in the range 300−2500 nm for a sample of 12 very low mass stars in the σ Orionis cluster. The sample includes eight stars with evidence of disks from Spitzer and four without disks, with masses ranging from 0.08 to 0.3 M . The aim of this first paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass young stars and the accuracy of the correlations between these secondary tracers (mainly accretion line luminosities) found in the literature. We use our spectra to measure the accretion luminosity from the continuum excess emission in the UV and visual; the derived mass accretion rates range from 10 −9 M yr −1 down to 5 × 10 −11 M yr −1 , allowing us to investigate the behavior of the accretion-driven emission lines in very low mass accretion rate regimes. We compute the luminosity of ten accretion-driven emission lines from the UV to the near-IR, which are all obtained simultaneously. In general, most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We recompute the relationships between the accretion luminosities and the line luminosities, and we confirm the validity of the correlations given in the literature, with the possible exception of Hα. Metallic lines, such as the CaII IR triplet or the Na I line at 589.3 nm, show a larger dispersion. When looking at individual objects, we find that the hydrogen recombination lines, from the UV to the near-IR, give good and consistent measurements of L acc that often better agree than the uncertainties introduced by the adopted correlations. The average L acc derived from several hydrogen lines, measured simultaneously, have a much reduced error. This suggests that some of the spread in the literature correlations may be due to the use of nonsimultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually.

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“…Second to the ultraviolet excess, U-band excesses are the best measure of the flux produced in the shock and have been shown to correlate with the total shock excess . However, at lowṀ emission at U may be dominated by chromospheric emission (Houdebine et al 1996;Franchini et al 1998); this chromospheric excess can confuse determinations of the accretion rate ). While possi-ble from the ground, U-band observations are still difficult to obtain, especially when extinction towards the source is relatively high as in the case of our sample, all with A V > 1.…”

Section: Accretion Ratesmentioning

confidence: 99%

On the Transitional Disk Class: Linking Observations of T Tauri Stars and Physical Disk Models

Espaillat

Ingleby

Hernández³

et al. 2012

ApJ

115

132

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Two decades ago "transitional disks" described spectral energy distributions (SEDs) of T Tauri stars with small near-IR excesses, but significant mid-and far-IR excesses. Many inferred this indicated dust-free holes in disks, possibly cleared by planets. Recently, this term has been applied disparately to objects whose Spitzer SEDs diverge from the expectations for a typical full disk. Here we use irradiated accretion disk models to fit the SEDs of 15 such disks in NGC 2068 and IC 348. One group has a "dip" in infrared emission while the others' continuum emission decreases steadily at all wavelengths. We find that the former have an inner disk hole or gap at intermediate radii in the disk and we call these objects "transitional" and "pre-transitional" disks, respectively. For the latter group, we can fit these SEDs with full disk models and find that millimeter data are necessary to break the degeneracy between dust settling and disk mass. We suggest the term "transitional" only be applied to objects that display evidence for a radical change in the disk's radial structure. Using this definition, we find that transitional and pre-transitional disks tend to have lower mass accretion rates than full disks and that transitional disks have lower accretion rates than pre-transitional disks. These reduced accretion rates onto the star could be linked to forming planets. Future observations of transitional and pre-transitional disks will allow us to better quantify the signatures of planet formation in young disks.

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Far‐Ultraviolet Continuum of G‐Type Stars: A Signature of the Temperature Minimum Region

Cited by 18 publications

References 8 publications

Chromospheric models of solar analogues with different activity levels

Chromospheric models of solar analogues with different activity levels

X-shooter spectroscopy of young stellar objects

On the Transitional Disk Class: Linking Observations of T Tauri Stars and Physical Disk Models

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