Using magnetic activity and Galactic dynamics to constrain the ages of M dwarfs

AbstractWe present a study of the dynamics and magnetic activity of M dwarfs using the largest spectroscopic sample of low-mass stars ever assembled. The age at which strong surface magnetic activity (as traced by Hα) ceases in M dwarfs has been inferred to have a strong dependence on mass (spectral type, surface temperature) and explains previous results showing a large increase in the fraction of active stars at later spectral types. Using spectral observations of more than 40000 M dwarfs from the Sloan Digital Sky Survey, we show that the fraction of active stars decreases as a function of vertical distance from the Galactic plane (a statistical proxy for age), and that the magnitude of this decrease changes significantly for different M spectral types. Adopting a simple dynamical model for thin disk vertical heating, we assign an age for the activity decline at each spectral type, and thus determine the activity lifetimes for M dwarfs. In addition, we derive a statistical age-activity relation for each spectral type using the dynamical model, the vertical distance from the Plane and the Hα emission line luminosity of each star (the latter of which also decreases with vertical height above the Galactic plane).

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Photometric flaring fraction of M dwarf stars from the SkyMapper Southern Survey

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2898 ◽

2019 ◽

Vol 491 (1) ◽

pp. 39-50 ◽

Cited By ~ 1

Author(s):

Seo-Won Chang ◽

Christian Wolf ◽

Christopher A Onken

Keyword(s):

Galactic Plane ◽

Search Algorithm ◽

M Dwarfs ◽

Dwarf Stars ◽

Vertical Distance ◽

Steep Decline ◽

Bona Fide ◽

Single Epoch ◽

M Dwarf Stars ◽

M Dwarf

ABSTRACT We present our search for flares from M dwarf stars in the SkyMapper Southern Survey DR1, which covers nearly the full Southern hemisphere with six-filter sequences that are repeatedly observed in the passbands uvgriz. This allows us to identify bona fide flares in single-epoch observations on time-scales of less than four minutes. Using a correlation-based outlier search algorithm we find 254 flare events in the amplitude range of Δu ∼ 0.1 to 5 mag. In agreement with previous work, we observe the flaring fraction of M dwarfs to increase from ∼30 to ∼1000 per million stars for spectral types M0 to M5. We also confirm the decrease in flare fraction with larger vertical distance from the Galactic plane which is expected from declining stellar activity with age. Based on precise distances from Gaia DR2, we find a steep decline in the flare fraction from the plane to 150 pc vertical distance and a significant flattening towards larger distances. We then reassess the strong type dependence in the flaring fraction with a volume-limited sample within a distance of 50 pc from the Sun: in this sample the trend disappears and we find instead a constant fraction of ∼1 650 per million stars for spectral types M1 to M5. Finally, large-amplitude flares with Δi > 1 mag are very rare with a fraction of ∼0.5 per million M dwarfs. Hence, we expect that M-dwarf flares will not confuse SkyMapper’s search for kilonovae from gravitational-wave events. proper references for those databases (or follow their guideline on citation).

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A homogeneous sample of 34 000 M7−M9.5 dwarfs brighter than J = 17.5 with accurate spectral types

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834591 ◽

2019 ◽

Vol 623 ◽

pp. A127 ◽

Cited By ~ 1

Author(s):

S. Ahmed ◽

S. J. Warren

Keyword(s):

Luminosity Function ◽

Galactic Plane ◽

Effective Area ◽

Sloan Digital Sky Survey ◽

M Dwarfs ◽

Homogeneous Sample ◽

Type Method ◽

Sky Survey ◽

Better Than

The space density of late M dwarfs, subtypes M7–M9.5, is not well determined. We applied the photo-type method to iz photometry from the Sloan Digital Sky Survey and YJHK photometry from the UKIRT Infrared Deep Sky Survey, over an effective area of 3070 deg2, to produce a new, bright J(Vega) < 17.5, homogeneous sample of 33 665 M7–M9.5 dwarfs. The typical S/N of each source summed over the six bands is > 100. Classifications are provided to the nearest half spectral subtype. Through a comparison with the classifications in the BOSS Ultracool Dwarfs (BUD) spectroscopic sample, the typing is shown to be accurately calibrated to the BUD classifications and the precision is better than 0.5 subtypes rms; i.e. the photo-type classifications are as precise as good spectroscopic classifications. Sources with large χ2 > 20 include several catalogued late-type subdwarfs. The new sample of late M dwarfs is highly complete, but there is a bias in the classification of rare peculiar blue or red objects. For example, L subdwarfs are misclassified towards earlier types by approximately two spectral subtypes. We estimate that this bias affects only ∼1% of the sources. Therefore the sample is well suited to measure the luminosity function and investigate the softening towards the Galactic plane of the exponential variation of density with height.

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A Deep Large area Search for Low Luminosity Stars

Highlights of Astronomy ◽

10.1017/s1539299600021675 ◽

1998 ◽

Vol 11 (1) ◽

pp. 435-435

Author(s):

Hugh R.A. Jones ◽

Mike R.S. Hawkins

Keyword(s):

Spectral Type ◽

Low Temperatures ◽

Current Model ◽

Brown Dwarfs ◽

Evolutionary Models ◽

Spectral Indices ◽

Late Type ◽

Brown Dwarf ◽

Large Area ◽

M Dwarfs

In a recent survey for faint red stars from a digital stack of Schmidt plates a number of candidate objects were identified. Parallax’s for three of these objects have been reported showing them to have luminosities which interpreted within the available evolutionary models indicate them to be good brown dwarf candidates. Here we examine spectra of these objects and others from the plate stack. Using standard spectral indices we find that for a given spectral type their spectra are more consistent with the Pleiades brown dwarfs (PPL 15, Teide 1 and Calar 3) than with standard late-type M dwarfs. Our interpretation is that this is due to their selection by RF IN colours which at values > 3 preferentially selects objects with relatively low gravities. For late-type M dwarfs and brown dwarfs low gravities are expected to be a reliable indication of youth. We also notice that the stack objects generally have strong FeH absorption for their spectral type. Current model atmospheres suggest that FeH strongly increases in strength toward lower metallicities and lower temperatures. We believe that this is not consistent with the available observational evidence from late-type M dwarfs. It is possible that solid Fe is forming inthe low temperature atmospheres relatively depleting FeH strengths toward lower temperatures. We find some evidence that for dwarfs at low temperatures dust formation is less prevalent in lower gravity objects suggesting that dwarfs at low temperatures stronger FeH may be an indication of youth. In addition to the spectral evidence the three stack objects whose parallax’s have been measured show small tangential velocities which is a further indication of youth.

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Eclipsing spotted giant star with K2 and historical photometry

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834106 ◽

2018 ◽

Vol 620 ◽

pp. A189 ◽

Cited By ~ 3

Author(s):

K. Oláh ◽

S. Rappaport ◽

T. Borkovits ◽

T. Jacobs ◽

D. Latham ◽

...

Keyword(s):

Binary System ◽

Main Sequence ◽

Rapid Evolution ◽

Magnetic Activity ◽

Primary Component ◽

Physical Parameters ◽

Giant Star ◽

Eclipsing Binary ◽

Giant Stars ◽

Active Stars

Context. Stars can maintain their observable magnetic activity from the pre-main sequence (PMS) to the tip of the red giant branch. However, the number of known active giants is much lower than active stars on the main sequence (MS) since the stars spend only about 10% of their MS lifetime on the giant branch. Due to their rapid evolution it is difficult to estimate the stellar parameters of giant stars. A possibility for obtaining more reliable stellar parameters for an active giant arises when it is a member of an eclipsing binary system. Aims. We have discovered EPIC 211759736, an active spotted giant star in an eclipsing binary system during the Kepler K2 Campaign 5. The eclipsing nature allows us to much better constrain the stellar parameters than in most cases of active giant stars. Methods. We have combined the K2 data with archival HATNet, ASAS, and DASCH photometry, new spectroscopic radial velocity measurements, and a set of follow-up ground-based BVRCIC photometric observations, to find the binary system parameters as well as robust spot models for the giant at two different epochs. Results. We determined the physical parameters of both stellar components and provide a description of the rotational and long-term activity of the primary component. The temperatures and luminosities of both components were examined in the context of the Hertzsprung–Russell diagram. We find that both the primary and the secondary components deviate from the evolutionary tracks corresponding to their masses in the sense that the stars appear in the diagram at lower masses than their true masses. Conclusions. We further evaluate the proposition that traditional methods generally result in higher masses for active giants than what is indicated by stellar evolution tracks in the HR diagram. A possible reason for this discrepancy could be a strong magnetic field, since we see greater differences in more active stars.

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Nonmigrating tidal signatures in the magnitude and the inter-hemispheric asymmetry of the equatorial ionization anomaly

Annales Geophysicae ◽

10.5194/angeo-31-1115-2013 ◽

2013 ◽

Vol 31 (6) ◽

pp. 1115-1130 ◽

Cited By ~ 24

Author(s):

C. Xiong ◽

H. Lühr

Keyword(s):

Hemispheric Asymmetry ◽

Strong Dependence ◽

Magnetic Activity ◽

Planetary Waves ◽

Leading Role ◽

Equatorial Ionization Anomaly ◽

June Solstice ◽

December Solstice ◽

Apex Height ◽

Stationary Planetary Wave

Abstract. Based on nine years of observations from the satellites CHAMP and GRACE the tidal signatures in the magnitude and the inter-hemisphere asymmetry of the equatorial ionization anomaly (EIA) have been investigated in this study. The EIA magnitude parameters show longitudinal wavenumber 4 and 3 (WN4/WN3) patterns during the months around August and December, respectively, while for different EIA parameters the contributions of the various tidal parameters are different. For the crest-to-trough ratio (CTR) the dominating nonmigrating tidal component contributing to WN4 is DE3 during the months around August, while during the months around December solstice the stationary planetary wave, SPW3, takes a comparable role to DE2 in contributing to WN3. For the apex height index (ApexHC) of the EIA fluxtube the stationary planetary waves, SPW4/SPW3, exceed the amplitudes of DE3/DE2 taking the leading role in causing the longitudinal WN4/WN3 patterns. During the months around December solstice the SW3 tide is prominent in both CTR and ApexHC. SW3 shows a strong dependence on the solar flux level, while it is hardly dependent on magnetic activity. For the EIA inter-hemispheric asymmetry only WN1 and WN2 longitudinal patterns can be seen. During June solstice months the pattern can be explained by stationary planetary waves SPW1 and SPW2. Conversely, around December solstice months longitudinal features exhibit some local time evolution, in particular the diurnal nonmigrating tide D0 takes the leading role.

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Magnetic activity in the HARPS M dwarf sample

Astronomy and Astrophysics ◽

10.1051/0004-6361/201527078 ◽

2017 ◽

Vol 600 ◽

pp. A13 ◽

Cited By ~ 59

Author(s):

N. Astudillo-Defru ◽

X. Delfosse ◽

X. Bonfils ◽

T. Forveille ◽

C. Lovis ◽

...

Keyword(s):

Monitoring Program ◽

Rotation Period ◽

Magnetic Activity ◽

Spectral Lines ◽

M Dwarfs ◽

Low Mass Stars ◽

The Galaxy ◽

Low Mass ◽

M Dwarf ◽

The Relationship

Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period (Prot) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the R'HK-index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs – the most numerous stars of the Galaxy – were left out of these analyses and no calibration of their Ca ii H and K emission to an R'HK exists to date. Aims. We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the R'HK-index that extends to the realm of M dwarfs, and by evaluating the relationship between R'HK and the rotation period. Methods. We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S-index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the R'HK. We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results. The R'HK index, like the fractional X-ray luminosity LX/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the R'HK index saturates to approximately 10-4. Stellar mass modulates the Ca ii activity, with R'HK showing a constant basal activity above 0.6 M⊙ and then decreasing with mass between 0.6 M⊙ and the fully-convective limit of 0.35 M⊙. Short-term variability of the activity correlates with its mean level and stars with higher R'HK indexes show larger R'HK variability, as previously observed for earlier spectral types.

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On the Accuracy of Spectral Classifications of Main-Sequence Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900099058 ◽

1973 ◽

Vol 50 ◽

pp. 52-59

Author(s):

W. Gliese

Keyword(s):

Spectral Type ◽

Main Sequence ◽

Standard Errors ◽

Main Sequence Stars ◽

M Dwarfs ◽

Spectral Class ◽

Classification Errors ◽

Nearby Stars

By examining the observed dispersion in (colour, spectral type) relations, classification errors have been derived from the data of nearby stars. The comparisons of the colour deviations observed in spectral regions of large variations of colour with type with the deviations in regions of small variations give the following standard errors in units of a tenth of a spectral class: For K dwarfs ±0.6 (MK), ±1.2 (Mt. Wilson), ±0.7 (Kuiper); for early M dwarfs ±0.9: (MK), ±0.7 (Mt. Wilson), ±0.5: (Kuiper); and for late M dwarfs ±0.7 (Kuiper).

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CHROMOSPHERIC VARIABILITY IN SLOAN DIGITAL SKY SURVEY M DWARFS. II. SHORT-TIMESCALE Hα VARIABILITY

The Astrophysical Journal ◽

10.1088/0004-637x/722/2/1352 ◽

2010 ◽

Vol 722 (2) ◽

pp. 1352-1359 ◽

Cited By ~ 23

Author(s):

E. A. Kruse ◽

E. Berger ◽

G. R. Knapp ◽

T. Laskar ◽

J. E. Gunn ◽

...

Keyword(s):

Sloan Digital Sky Survey ◽

M Dwarfs ◽

Short Timescale ◽

Sky Survey

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Modelling of an Eclipsing RS CVn Binary: V405 And

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311027347 ◽

2011 ◽

Vol 7 (S282) ◽

pp. 199-200

Author(s):

Krisztián Vida ◽

Katalin Oláh ◽

Zsolt Kővári

Keyword(s):

Light Curve ◽

Main Sequence ◽

Magnetic Activity ◽

Light Curves ◽

Stellar Structure ◽

Main Sequence Stars ◽

M Dwarfs ◽

Primary Star ◽

Convective Envelope ◽

Eclipsing Binary

AbstractV405 And is an ultrafast-rotating (Prot ≈ 0.46 days) eclipsing binary. The system consists of a primary star with radiative core and convective envelope, and a fully convective secondary. Theories have shown that stellar structure can depend on magnetic activity, i.e., magnetically active M-dwarfs should have larger radii. Earlier light curve modelling of V405 And indeed showed this behaviour: we found that the radius of the primary is significantly larger than the theoretically predicted value for inactive main sequence stars (the discrepancy is the largest of all known objects), while the secondary fits well to the mass-radius relation. By modelling our recently obtained light curves, which show significant changes of the spotted surface of the primary, we can find further proof for this phenomenon.

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Do Low Luminosity Stars Matter?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310008185 ◽

2009 ◽

Vol 5 (H15) ◽

pp. 47-60

Author(s):

María Teresa Ruiz

Keyword(s):

White Dwarf ◽

Galactic Plane ◽

M Dwarfs ◽

Low Mass Stars ◽

Missing Mass ◽

Low Mass ◽

Astronomical Research ◽

Red Giant ◽

Large Telescopes ◽

The Universe

AbstractHistorically, low luminosity stars have attracted very little attention, in part because they are difficult to see except with large telescopes, however, by neglecting to study them we are leaving out the vast majority of stars in the Universe. Low mass stars evolve very slowly, it takes them trillions of years to burn their hydrogen, after which, they just turn into a He white dwarf, without ever going through the red giant phase. This lack of observable evolution partly explains the lack of interest in them. The search for the “missing mass” in the galactic plane turned things around and during the 60s and 70s the search for large M/L objects placed M-dwarfs and cool WDs among objects of astrophysical interest. New fields of astronomical research, like BDs and exoplanets appeared as spin-offs from efforts to find the “missing mass”. The search for halo white dwarfs, believed to be responsible for the observed microlensing events, is pursued by several groups. The progress in these last few years has been tremendous, here I present highlights some of the great successes in the field and point to some of the still unsolved issues.

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