Observation and modelling of main sequence star chromospheres

Houdebine, E. R.; Junghans, K.; Heanue, M. C.; Andrews, A. D.

doi:10.1051/0004-6361/200811349

Cited by 14 publications

(12 citation statements)

References 77 publications

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“…In addition, T Tauri stars possess largely convective interiors while moving from the birth line towards the zero age main sequence (ZAMS) so that they are able to generate kiloGauss magnetic fields [43]. As a result, T Tauri stars show all signs of enhanced magnetic activity in the form of strong chromospheric and coronal emission, in comparison with their main-sequence siblings [44,45].…”

Section: Stellar Uv Emission Of Cttss From Magnetic Activitymentioning

confidence: 99%

The UV Perspective of Low-Mass Star Formation

Schneider¹,

Günther²

2020

Galaxies

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The formation of low-mass (M 2 M ) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited to study star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows.CTTSs were initially identified as a new class of objects due their optical variability. Very early on, it was realized that they also show signs of enhanced chromospheric activity, i.e., "emission lines resembling those of the solar chromosphere" [2]. These emission lines are superposed on a photospheric spectrum similar to main sequence stars of late spectral type. Initially, the source of emission in excess of a normal photosphere was speculated to be circumstellar or chromospheric in origin [3].The discovery that CTTSs are above and to the right of the main sequence in an HR-diagram demonstrated in conjunction with theoretical work [4], that these objects must be young and, thus, the progenitors of the large main sequence population [5,6]. This notion is corroborated by the presence of strong Li absorption, which requires ∼ 100× the Li abundance of the Sun [e.g. 7]. Because Li is depleted quickly in stellar interiors, the amount of surface lithium decreases with time in convective stars. Therefore, strong Li absorption can only be found in young stars and thus CTTS must be young [e.g. review by 8].Meanwhile the number of peculiarities found in CTTSs grew, but their established youth alone was insufficient to explain those peculiarities and the "mysteries" of CTTSs remained. Many features, like the origin of the "chromospheric emission" remained unexplained, until the early 1980s when a large variety of models were proposed to explain features of CTTSs including envelopes, outflows, infall, and dust disks.Specifically, the "ultraviolet excess" (measured around 3700 Å) and the "blue continuum", which go along with a weakening of photosperic absorption lines due to veiling [9,10], were shown to correlate with the strength of Hα [11,12]. Such a correlation indicates a common origin of both features as observed for chromospheric emission on the Sun. Thus, it was natural to ascribe the excess continuum emission at short wavelengths to a Balmer continuum, i.e., emission following the capture of a free electron by an ionized hydrogen atom. While that finding pinpoints neither the physical origin nor location of the emitting material, it paved the way for explanations involving a suitably tuned chromosphere for producing the observed (Balmer & Ca II H+K) line fluxes in CTTSs. Quantitative mod...

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Section: Stellar Uv Emission Of Cttss From Magnetic Activitymentioning

confidence: 99%

The UV Perspective of Low-Mass Star Formation

Schneider¹,

Günther²

2020

Galaxies

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“…In addition to Hα, other lines in the range of the visible channel of CARMENES that have been found to have chromospheric components sensitive to activity are the He I D 3 (hereafter He D 3 ) line, the Na I D (hereafter Na D) doublet, and the Ca II infrared triplet (hereafter Ca IRT; e.g. Gizis et al 2002;Houdebine et al 2009;Robertson et al 2016;Martin et al 2017). With the near-infrared channel, the He I λ10833 Å (hereafter He 10833) and Paβ lines are accessible.…”

Section: Introductionmentioning

confidence: 99%

The CARMENES search for exoplanets around M dwarfs

Schöfer¹,

Jeffers²,

Reiners³

et al. 2019

A&A

112

133

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Context. The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES) survey is searching for Earth-like planets orbiting M dwarfs using the radial velocity method. Studying the stellar activity of the target stars is important to avoid false planet detections and to improve our understanding of the atmospheres of late-type stars. Aims. In this work we present measurements of activity indicators at visible and near-infrared wavelengths for 331 M dwarfs observed with CARMENES. Our aim is to identify the activity indicators that are most sensitive and easiest to measure, and the correlations among these indicators. We also wish to characterise their variability. Methods. Using a spectral subtraction technique, we measured pseudo-equivalent widths of the He I D3, Hα, He I λ10833 Å, and Pa β lines, the Na I D doublet, and the Ca II infrared triplet, which have a chromospheric component in active M dwarfs. In addition, we measured an index of the strength of two TiO and two VO bands, which are formed in the photosphere. We also searched for periodicities in these activity indicators for all sample stars using generalised Lomb-Scargle periodograms. Results. We find that the most slowly rotating stars of each spectral subtype have the strongest Hα absorption. Hα is correlated most strongly with He I D3, whereas Na I D and the Ca II infrared triplet are also correlated with Hα. He I λ10833 Å and Paβ show no clear correlations with the other indicators. The TiO bands show an activity effect that does not appear in the VO bands. We find that the relative variations of Hα and He I D3 are smaller for stars with higher activity levels, while this anti-correlation is weaker for Na I D and the Ca II infrared triplet, and is absent for He I λ10833 Å and Paβ. Periodic variation with the rotation period most commonly appears in the TiO bands, Hα, and in the Ca II infrared triplet.

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“…Interference fringes are usually reasonably well corrected by the flat‐field (e.g. Houdebine et al 2009) even when their amplitude is large such as for the CES (ESO). As for scattered light, our experience is that it has negligible effects at about 6500 Å and may be at most a few per cent of the stellar light.…”

Section: Resultsmentioning

confidence: 99%

Observation and modelling of main-sequence star chromospheres - XVII. Rotation of dM4 stars★

Houdebine¹

2012

Monthly Notices of the Royal Astronomical Society

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Using two different spectrographs, High Accuracy Radial velocity Planet Searcher (European Southern Observatory) and SOPHIE (Observatoire de Haute Provence), we have measured v sin i for a sample of 23 dM4 stars. These are the first measurements of v sin i for most of the stars studied here. We measured v sin i to a precision of 0.3 km s−1 and a detection limit of about 0.5–1 km s−1. All our targets have similar (R−I)C colour. This is an advantage and facilitates the determination of the narrowest cross‐correlation profiles for v sin i∼ 0. In our sample, we detected rotation for 21 stars (12 dM4e stars and nine dM4 stars) and we did not detect rotation in a further two stars. This result shows that there are many dM4 fast rotators (of the order of 5 d), and many more than in the case of dM1 stars. We determined radii and effective temperatures for all our target stars. The effective temperatures were derived using the (R−I)C colour and empirical far‐red colour–effective temperature correlations. We derived the radii from the standard formulae relating Mbol, bolometric correction (BC) and Teff. We find that the distribution of P/sin i (the projected rotation period) is bimodal with a maximum for slow rotators around 14 d and for fast rotators around 6 d, similar to dM1 stars. The rotation period appears to decrease with decreasing radius, both among dM4 and dM4e stars. The same finding was obtained in our previous study of dM1 stars.

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Observation and modelling of main sequence star chromospheres

Cited by 14 publications

References 77 publications

The UV Perspective of Low-Mass Star Formation

The UV Perspective of Low-Mass Star Formation

The CARMENES search for exoplanets around M dwarfs

Observation and modelling of main-sequence star chromospheres - XVII. Rotation of dM4 stars★

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