Evolution of magnetic activity in intermediate-mass giants

AbstractThe X-ray surface fluxes of intermediate-mass G and K giants are correlated with their rotation periods and Rossby numbers. Empirical relationships are presented that accounts for the X-ray luminosity evolution of single intermediate-mass giants, such as FK Comae-type stars, and of giants in close or long-period binaries, such as RS CVn-type systems, as they evolve off the main sequence towards the top of the red giant branch.

Download Full-text

On the relation between rotation, convection and activity in intermediate-mass giants

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307000324 ◽

2006 ◽

Vol 2 (S239) ◽

pp. 154-156

Author(s):

P. Gondoin

Keyword(s):

Binary Systems ◽

Main Sequence ◽

Empirical Relation ◽

Rotation Period ◽

Surface Flux ◽

Type Systems ◽

Intermediate Mass ◽

X Ray ◽

Red Giant ◽

Luminosity Evolution

AbstractI found evidence that the X-ray surface flux of intermediate-mass G and K giants is correlated with their rotation period and Rossby number. Confidence in the degree of correlation is significantly higher when stellar gravity is taken into account. An empirical relation is found that accounts for the X-ray luminosity evolution of single intermediate-mass giants and giants in close or long-period binary systems, such as RS CVn-type systems, as they evolve off the main sequence towards the top of red giant branch.

Download Full-text

Activity and rotation of the X-ray emitting Kepler stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731674 ◽

2019 ◽

Vol 628 ◽

pp. A41 ◽

Cited By ~ 2

Author(s):

D. Pizzocaro ◽

B. Stelzer ◽

E. Poretti ◽

S. Raetz ◽

G. Micela ◽

...

Keyword(s):

White Light ◽

Main Sequence ◽

Magnetic Activity ◽

Light Curves ◽

Late Type ◽

Main Sequence Stars ◽

Photometric Variability ◽

X Ray ◽

Rotation Periods ◽

Flare Frequency

The relation between magnetic activity and rotation in late-type stars provides fundamental information on stellar dynamos and angular momentum evolution. Rotation-activity studies found in the literature suffer from inhomogeneity in the measurement of activity indexes and rotation periods. We overcome this limitation with a study of the X-ray emitting, late-type main-sequence stars observed by XMM-Newton and Kepler. We measured rotation periods from photometric variability in Kepler light curves. As activity indicators, we adopted the X-ray luminosity, the number frequency of white-light flares, the amplitude of the rotational photometric modulation, and the standard deviation in the Kepler light curves. The search for X-ray flares in the light curves provided by the EXTraS (Exploring the X-ray Transient and variable Sky) FP-7 project allows us to identify simultaneous X-ray and white-light flares. A careful selection of the X-ray sources in the Kepler field yields 102 main-sequence stars with spectral types from A to M. We find rotation periods for 74 X-ray emitting main-sequence stars, 20 of which do not have period reported in the previous literature. In the X-ray activity-rotation relation, we see evidence for the traditional distinction of a saturated and a correlated part, the latter presenting a continuous decrease in activity towards slower rotators. For the optical activity indicators the transition is abrupt and located at a period of ~10 d but it can be probed only marginally with this sample, which is biased towards fast rotators due to the X-ray selection. We observe seven bona-fide X-ray flares with evidence for a white-light counterpart in simultaneous Kepler data. We derive an X-ray flare frequency of ~0.15 d−1, consistent with the optical flare frequency obtained from the much longer Kepler time-series.

Download Full-text

Spectropolarimetric follow-up of 8 rapidly rotating, X-ray bright FK Comae candidates

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1455 ◽

2020 ◽

Vol 496 (1) ◽

pp. 295-308

Author(s):

J Sikora ◽

J Rowe ◽

S B Howell ◽

E Mason ◽

G A Wade

Keyword(s):

Evolutionary History ◽

Main Sequence ◽

Magnetic Activity ◽

Single Star ◽

X Ray ◽

Spectroscopic Binary ◽

Short Period ◽

Show Evidence ◽

Red Giant

ABSTRACT Our understanding of the evolved, rapidly rotating, magnetically active, and apparently single FK Comae stars is significantly hindered by their extreme rarity: only two stars in addition to FK Com itself are currently considered to be members of this class. Recently, a sample of more than 20 candidate FK Comae type stars was identified within the context of the Kepler–Swift Active Galaxies and Stars (KSwAGS) survey. We present an analysis of high-resolution Stokes V observations obtained using ESPaDOnS@CFHT for 8 of these candidates. We found that none of these targets can be considered members of the FK Comae class based primarily on their inferred rotational velocities and on the detection of spectroscopic binary companions. However, 2 targets show evidence of magnetic activity and have anomalously high projected rotational velocities (vsin i) relative to typical values associated with stars of similar evolutionary states. EPIC 210426551 has a $v\sin {i}=209\, {\rm km\, s}^{-1}$, an estimated mass of $1.07\, \mathrm{ M}_\odot$, and, based in part on its derived metallicity of [M/H] = −0.4, it is either an evolved main sequence (MS) star or a pre-MS star. KIC 7732964 has a mass of $0.84\, \mathrm{ M}_\odot$, lies near the base of the red giant branch, and exhibits a $v\sin {i}=23\, {\rm km\, s}^{-1}$. We find that these two objects have similar characteristics to FK Com (albeit less extreme) and that their rapid rotation may be inconsistent with that predicted for a single star evolutionary history. Additional observations are necessary in order to better constrain their evolutionary states and whether they have short-period binary companions.

Download Full-text

Updated X-ray view of the Hyades cluster

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937304 ◽

2020 ◽

Vol 640 ◽

pp. A66 ◽

Cited By ~ 1

Author(s):

S. Freund ◽

J. Robrade ◽

P. C. Schneider ◽

J. H. M. M. Schmitt

Keyword(s):

Main Sequence ◽

Distribution Functions ◽

X Rays ◽

Stellar Rotation ◽

Detection Rates ◽

X Ray ◽

Rotation Periods ◽

Hyades Cluster ◽

Bona Fide ◽

Tidal Tails

Aims. We revisit the X-ray properties of the main sequence Hyades members and the relation between X-ray emission and stellar rotation. Methods. As an input catalog for Hyades members, we combined three recent Hyades membership lists derived from Gaia DR2 data that include the Hyades core and its tidal tails. We searched for X-ray detections of the main sequence Hyades members in the ROSAT all-sky survey, and pointings from ROSAT, the Chandra X-Ray Observatory, and XMM-Newton. Furthermore, we adopted rotation periods derived from Kepler’s K2 mission and other resources. Results. We find an X-ray detection for 281 of 1066 bona fide main sequence Hyades members and provide statistical upper limits for the undetected sources. The majority of the X-ray detected stars are located in the Hyades core because of its generally smaller distance to the Sun. F- and G-type stars have the highest detection fraction (72%), while K- and M-type dwarfs have lower detection rates (22%). The X-ray luminosities of the detected members range from ∼2 × 1027 erg s−1 for late M-type dwarfs to ∼2 × 1030 erg s−1 for active binaries. The X-ray luminosity distribution functions formally differ for the members in the core and tidal tails, which is likely caused by a larger fraction of field stars in our Hyades tails sample. Compared to previous studies, our sample is slightly fainter in X-rays due to differences in the Hyades membership list used; furthermore, we extend the X-ray luminosity distribution to fainter luminosities. The X-ray activity of F- and G-type stars is well defined at FX/Fbol ≈ 10−5. The fractional X-ray luminosity and its spread increases to later spectral types reaching the saturation limit (FX/Fbol ≈ 10−3) for members later than spectral type M3. Confirming previous results, the X-ray flux varies by less than a factor of three between epochs for the 104 Hyades members with multiple epoch data, significantly less than expected from solar-like activity cycles. Rotation periods are found for 204 Hyades members, with about half of them being detected in X-rays. The activity-rotation relation derived for the coeval Hyades members has properties very similar to those obtained by other authors investigating stars of different ages.

Download Full-text

The effects of rotation on the lithium depletion of G- and K-dwarfs in Messier 35

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3141 ◽

2020 ◽

Vol 500 (1) ◽

pp. 1158-1177

Author(s):

R D Jeffries ◽

R J Jackson ◽

Qinghui Sun ◽

Constantine P Deliyannis

Keyword(s):

Rotation Rate ◽

Binary Systems ◽

Main Sequence ◽

Strong Relationship ◽

Magnetic Activity ◽

Rich Cluster ◽

Stellar Radius ◽

Rotation Periods ◽

Probability Zero ◽

Lithium Depletion

ABSTRACT New fibre spectroscopy and radial velocities from the WIYN telescope are used to measure photospheric lithium in 242 high-probability, zero-age main-sequence F- to K-type members of the rich cluster M35. Combining these with published rotation periods, the connection between lithium depletion and rotation is studied in unprecedented detail. At Teff < 5500 K there is a strong relationship between faster rotation and less Li depletion, although with a dispersion larger than measurement uncertainties. Components of photometrically identified binary systems follow the same relationship. A correlation is also established between faster rotation rate (or smaller Rossby number), decreased Li depletion and larger stellar radius at a given Teff. These results support models where star-spots and interior magnetic fields lead to inflated radii and reduced Li depletion during the pre-main-sequence (PMS) phase for the fastest rotators. However, the data are also consistent with the idea that all stars suffered lower levels of Li depletion than predicted by standard PMS models, perhaps because of deficiencies in those models or because saturated levels of magnetic activity suppress Li depletion equally in PMS stars of similar Teff regardless of rotation rate, and that slower rotators subsequently experience more mixing and post-PMS Li depletion.

Download Full-text

X-ray emission regimes and rotation sequences in the M34 open cluster

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314001823 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 106-109

Author(s):

Philippe Gondoin

Keyword(s):

Main Sequence ◽

Early Stage ◽

Open Cluster ◽

Magnetic Activity ◽

X Ray

AbstractI report on a correlation between the saturated and non-saturated regimes of X-ray emission and the rotation sequences that have been observed in the M34 open cluster. An interpretation of this correlation in term of magnetic activity evolution in the early stage of evolution on the main sequence is presented.

Download Full-text

X-Ray Properties of Pre-Main-Sequence Stars in the Orion Nebula Cluster with Known Rotation Periods

The Astronomical Journal ◽

10.1086/420989 ◽

2004 ◽

Vol 127 (6) ◽

pp. 3537-3552 ◽

Cited By ~ 56

Author(s):

Keivan G. Stassun ◽

David R. Ardila ◽

Mary Barsony ◽

Gibor Basri ◽

Robert D. Mathieu

Keyword(s):

Main Sequence ◽

Orion Nebula ◽

Main Sequence Stars ◽

X Ray ◽

Rotation Periods

Download Full-text

Non thermal emission from T Tauri stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310016492 ◽

2010 ◽

Vol 6 (S275) ◽

pp. 404-405

Author(s):

María V. del Valle ◽

Gustavo E. Romero

Keyword(s):

Main Sequence ◽

Thermal Emission ◽

High Energy ◽

Magnetic Activity ◽

T Tauri Stars ◽

Main Sequence Stars ◽

High Energy Radiation ◽

X Ray ◽

T Tauri ◽

Low Mass

AbstractT Tauri stars are low mass, pre-main sequence stars. These objects are surrounded by an accretion disk and present strong magnetic activity. T Tauri stars are copious emitters of X-ray emission which belong to powerful magnetic reconnection events. Strong magnetospheric shocks are likely outcome of massive reconnection. Such shocks can accelerate particles up to relativistic energies through Fermi mechanism. We present a model for the high-energy radiation produced in the environment of T Tauri stars. We aim at determining whether this emission is detectable. If so, the T Tauri stars should be very nearby.

Download Full-text

Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833498 ◽

2018 ◽

Vol 618 ◽

pp. A48 ◽

Cited By ~ 12

Author(s):

M. Mittag ◽

J. H. M. M. Schmitt ◽

K.-P. Schröder

Keyword(s):

Main Sequence ◽

Rotation Period ◽

Magnetic Activity ◽

Turnover Time ◽

Rossby Number ◽

Stellar Rotation ◽

Main Sequence Stars ◽

Rotation Periods ◽

Period Distribution ◽

Cool Stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R+HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

Download Full-text