Are Magnetically Active Low‐Mass M Dwarfs Completely Convective?

Mullan, D. J.; MacDonald, J.

doi:10.1086/322336

Cited by 229 publications

(286 citation statements)

References 49 publications

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“…Spot coverage can produce similar effects. Theoretical and observational evidence for the conservation of the luminosity in these systems has been presented by Delfosse et al (2000), Mullan & MacDonald (2001), Torres & Ribas (2002), Ribas (2006), Torres et al (2006), Chabrier et al (2007), and others (see also Morales et al 2008). We do not see any obvious discrepancy in the temperature of CV Boo B compared to models, although our uncertainties are large enough that the effect may be masked.…”

Section: Discussionmentioning

confidence: 84%

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Absolute Properties of the Spotted Eclipsing Binary Star Cv Boötis

Torres

Vaz

Lacy

2008

The Astronomical Journal

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We present new V-band differential brightness measurements as well as new radial-velocity measurements of the detached, circular, 0.84 day period, double-lined eclipsing binary system, CV Boo. These data, along with other observations from the literature, are combined to derive improved absolute dimensions of the stars for the purpose of testing various aspects of theoretical modeling. Despite complications from intrinsic variability that we detect in the system, and despite the rapid rotation of the components, we are able to determine the absolute masses and radii to better than 1.3% and 2%, respectively. We obtain M A = 1.032 ± 0.013 M and R A = 1.262 ± 0.023 R for the hotter, larger, and more-massive primary (star A), and M B = 0.968 ± 0.012 M and R B = 1.173 ± 0.023 R for the secondary. The estimated effective temperatures are 5760 ± 150 K and 5670 ± 150 K, respectively. The intrinsic variability with a period ∼1% shorter than the orbital period is interpreted as being due to modulation by spots on one or both components. This implies that the spotted star(s) must be rotating faster than the synchronous rate, which disagrees with predictions from current tidal evolution models according to which both stars should be synchronized. We also find that the radius of the secondary is larger than expected from stellar evolution calculations by ∼10%, a discrepancy also seen in other (mostly lower-mass and active) eclipsing binaries. We estimate the age of the system to be approximately 9 Gyr. Both components are near the end of their main-sequence phase, and the primary may have started the shell hydrogen-burning stage.

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Section: Discussionmentioning

confidence: 84%

“…systems (see, e.g., Mullan & MacDonald 2001;Chabrier et al 2007, for the theoretical context). The signs of activity in CV Boo are fairly obvious (spottedness, X-ray emission), and are no doubt associated with the rapid rotation of the components.…”

Section: Comparison With Stellar Evolution Theorymentioning

confidence: 99%

Absolute Properties of the Spotted Eclipsing Binary Star Cv Boötis

Torres

Vaz

Lacy

2008

The Astronomical Journal

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“…Note also that Mullan & MacDonald (2001) have presented evidence that magnetically active, but single, M dwarfs have signiÐcantly larger radii than inactive dM stars of the same e †ective temperature. This may well be another manifestation of the phenomenon seen in V471 Tau.…”

Section: T He Stellar Masses and Orbital Parametersmentioning

confidence: 93%

Hubble Space TelescopeSpectroscopy of V471 Tauri: Oversized K Star, Paradoxical White Dwarf

O'Brien

Bond

Sion

2001

ApJ

110

106

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We have used the Goddard High Resolution Spectrograph on board the Hubble Space T elescope to obtain Lya spectra of the hot white dwarf (WD) component of the short-period eclipsing DA]dK2 precataclysmic binary V471 Tauri, a member of the Hyades star cluster. Radial velocities of the WD were determined from eight post-COSTAR spectra obtained near the two quadratures of the orbit. When combined with ground-based measurements of the dK velocities, eclipse timings, and a determination of the dK starÏs rotational velocity, the data constrain the orbital inclination to be i \ 77¡ and yield dynamical masses for the components of and Model atmosphere M WD \ 0.84 M dK \ 0.93 M _ . Ðtting of the Lya proÐle provides the e †ective temperature (34,500 K) and surface gravity (log g \ 8.3) of the WD. The radius of the dK component is about 18% larger than that of a normal Hyades dwarf of the same mass. This expansion is attributed to the large degree of coverage of the stellar surface by starspots, which is indicated by both radiometric measurements and ground-based Doppler imaging ; in response, the star has expanded in order to maintain the luminosity of a 0.93 dwarf. The radius of M _ the WD, determined from a radiometric analysis and from eclipse ingress timings, is 0.0107 The R _ . position of the star in the mass-radius plane is in full accord with theoretical predictions for a degenerate carbon-oxygen WD with a surface temperature equal to that observed. The position of the WD in the H-R diagram is also fully consistent with that expected for a WD with our dynamically measured mass. Both comparisons with theory are probably the most stringent yet made for any WD. The theoretical cooling age of the WD is 107 yr. The high e †ective temperature and high mass of the WD present an evolutionary paradox. The WD is the most massive one known in the Hyades but also the hottest and youngest, in direct conÑict with expectation. We examine possible resolutions of the paradox, including the possibility of a nova outburst in the recent past, but conclude that the most likely explanation is that the WD is indeed very young and is descended from a blue straggler. A plausible scenario is that the progenitor system was a triple, with a close inner pair of main-sequence stars whose masses were both similar to that of the present cluster turno †. These stars became an Algol-type binary, which merged after several hundred million years to produce a single blue straggler of about twice the turno † mass. When this star evolved to the asymptotic giant branch phase, it underwent a common envelope interaction with a distant dK companion, which spiraled down to its present separation, and ejected the envelope. We estimate that the common envelope efficiency parameter was on the order of 0.3È1.0, in a CE good agreement with recent hydrodynamical simulations.

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“…There is growing observational evidence from fast-rotating, magnetically active stars in tidally-locked eclipsing binaries, young clusters and the field, that magnetic activity may increase the radii of low-mass stars (Lopez-Morales 2007; Morales, Ribas & Jordi 2008;Jackson, Jeffries & Maxted 2009;Stassun et al 2012). The mechanism by which it does so is still unclear; but could include the magnetic stabilisation of the star against convection (Gough & Tayler 1966;Moss 1968;Mullan & Macdonald 2001;Feiden & Chaboyer 2013), a reduction in convective efficiency due to a turbulent dynamo (Feiden & Chaboyer 2014) or the blocking of emergent flux by dark, magnetic starspots (Spruit 1982;Spruit & Weiss 1986;Jackson & Jeffries 2014). If PMS stellar radii are increased by magnetic activity, they could have lower central temperatures and hence less Li depletion at a given age.…”

Section: Introductionmentioning

confidence: 99%

The effect of star-spots on the ages of low-mass stars determined from the lithium depletion boundary

Jackson

Jeffries

2014

Monthly Notices of the Royal Astronomical Society

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In a coeval group of low-mass stars, the luminosity of the sharp transition between stars that retain their initial lithium and those at slightly higher masses in which Li has been depleted by nuclear reactions, the lithium depletion boundary (LDB), has been advanced as an almost model-independent means of establishing an age scale for young stars. Here we construct polytropic models of contracting pre-main sequence stars (PMS) that have cool, magnetic starspots blocking a fraction β of their photospheric flux. Starspots slow the descent along Hayashi tracks, leading to lower core temperatures and less Li destruction at a given mass and age. The age, τ LDB , determined from the luminosity of the LDB, L LDB , is increased by a factor (1 − β) −E compared to that inferred from unspotted models, where E ≃ 1 + d log τ LDB /d log L LDB and has a value ∼ 0.5 at ages < 80 Myr, decreasing to ∼ 0.3 for older stars. Spotted stars have virtually the same relationship between K-band bolometric correction and colour as unspotted stars, so this relationship applies equally to ages inferred from the absolute K magnitude of the LDB. Low-mass PMS stars do have starspots, but the appropriate value of β is highly uncertain with a probable range of 0.1 < β < 0.4. For the smaller β values our result suggests a modest systematic increase in LDB ages that is comparable with the maximum levels of theoretical uncertainty previously claimed for the technique. The largest β values would however increase LDB ages by 20-30 per cent and demand a re-evaluation of other age estimation techniques calibrated using LDB ages.

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Are Magnetically Active Low‐Mass M Dwarfs Completely Convective?

Cited by 229 publications

References 49 publications

Absolute Properties of the Spotted Eclipsing Binary Star Cv Boötis

Absolute Properties of the Spotted Eclipsing Binary Star Cv Boötis

Hubble Space TelescopeSpectroscopy of V471 Tauri: Oversized K Star, Paradoxical White Dwarf

The effect of star-spots on the ages of low-mass stars determined from the lithium depletion boundary

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