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

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.

scholarly journals Evolution of magnetic activity in intermediate-mass giants

2013 ◽  
Vol 9 (S302) ◽  
pp. 377-378
Author(s):  
Philippe Gondoin
Keyword(s):  
Main Sequence ◽  
Type Systems ◽  
Magnetic Activity ◽  
Surface Fluxes ◽  
Intermediate Mass ◽  
Empirical Relationships ◽  
X Ray ◽  
Rotation Periods ◽  
Red Giant ◽  
Luminosity Evolution

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.

scholarly journals Search for dormant black holes in ellipsoidal variables III. The OGLE BULGE short-period sample

2021 ◽  
Author(s):  
Roy Gomel ◽  
Simchon Faigler ◽  
Tsevi Mazeh ◽  
Michał Pawlak
Keyword(s):  
Main Sequence ◽  
Compact Object ◽  
Close Binaries ◽  
Periodic Modulation ◽  
Mass Ratio ◽  
True Nature ◽  
The Third ◽  
Short Period ◽  
Actual Mass

Abstract This is the third of a series of papers that presents an algorithm to search for close binaries with massive, possibly compact, unseen secondaries. The detection of such a binary is based on identifying a star that displays a large ellipsoidal periodic modulation, induced by tidal interaction with its companion. In the second paper of the series we presented a simple approach to derive a robust modified minimum mass ratio (mMMR), based on the observed ellipsoidal amplitude, without knowing the primary mass and radius, assuming the primary fills its Roche lobe. The newly defined mMMR is always smaller than the actual mass ratio. Therefore, a binary with an mMMR larger than unity is a good candidate for having a massive secondary, which might be a black hole or a neutron star. This paper considers 10,956 OGLE short-period ellipsoidals observed towards the Galactic Bulge. We re-analyse their modulation and identify 136 main-sequence systems with mMMR significantly larger than unity as candidates for having compact-object secondaries, assuming their observed periodic modulations reflect indeed the ellipsoidal effect. Obviously, one needs follow-up observations to find out the true nature of these companions.

scholarly journals Activity and rotation of the X-ray emitting Kepler stars

2019 ◽  
Vol 628 ◽  
pp. A41 ◽  
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.

scholarly journals TESS observations of pulsating subdwarf B stars: extraordinarily short-period gravity modes in CD−28° 1974

2020 ◽  
Vol 493 (4) ◽  
pp. 5162-5169 ◽  
Author(s):  
M D Reed ◽  
K A Shoaf ◽  
P Németh ◽  
J Vos ◽  
M Uzundag ◽  
...  
Keyword(s):  
Main Sequence ◽  
Orbital Motion ◽  
Proper Motions ◽  
Reflection Effect ◽  
Echelle Spectrograph ◽  
Single Star ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Short Period ◽  
Typical Period

ABSTRACT Transiting Exoplanet Survey Satellite (TESS) observations show CD−28° 1974 to be a gravity(g)-mode-dominated hybrid pulsating subdwarf B (sdBV) star. It shows 13 secure periods that form an ℓ = 1 asymptotic sequence near the typical period spacing. Extraordinarily, these periods lie between 1500 and 3300 s, whereas typical $\ell = 1\, g$ modes in sdBV stars occur between 3300 and 10 000 s. This indicates a structure somewhat different from typical sdBV stars. CD−28° 1974 has a visually close F/G main-sequence companion 1.33 arcsec away, which may be a physical companion. Gaia proper motions indicate a comoving pair with the same distance. A reanalysis of Ultraviolet and Visual Echelle Spectrograph (UVES) spectra failed to detect any orbital motion and the light curve shows no reflection effect or ellipsoidal variability, making an unseen close companion unlikely. The implication is that CD−28° 1974 has become a hot subdwarf via single star or post-merger evolution.

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

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.

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

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.

scholarly journals Non thermal emission from T Tauri stars

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.

scholarly journals The Spectroscopic Binary Characteristics of the Mercury-Manganese Stars

1976 ◽  
Vol 32 ◽  
pp. 533
Author(s):  
G.C.L. Aikman
Keyword(s):  
Main Sequence ◽  
High Dispersion ◽  
Orbital Elements ◽  
Circular Orbits ◽  
Spectroscopic Binary ◽  
Weak Evidence ◽  
Short Period

SummaryThe 80 confirmed and suspected mercury manganese stars identified prior to 1974 and north of declination -20° have been studied from 654 high dispersion spectrograms. (2.4 - 15 Å mm-1) for velocity and degree of peculiarity. The binary frequency based on velocity variability criteria alone appears to be a normal main sequence value of about 50%, and is probably independent of the degree of enhancement of the mercury and manganese lines. New spectroscopic orbital elements are given for six systems: α And, 2 Per, κ Cnc, φ Her, HR 6620 and 46 Dra. The Hg Mn spectrosopic binaries appear to be fairly normal in their orbital characteristics, except that periods less than 3 days are conspicuously absent; there is also weak evidence for more nearly circular orbits amongst the short period Hg Mn binaries than in comparable normal binaries.

scholarly journals Structures of Binary Star Coronae

2004 ◽  
Vol 219 ◽  
pp. 199-210
Author(s):  
Nancy S. Brickhouse
Keyword(s):  
Binary Systems ◽  
Binary Star ◽  
Magnetic Loop ◽  
Close Binaries ◽  
Coronal Emission ◽  
Single Star ◽  
X Ray ◽  
Radio Studies ◽  
Show Evidence ◽  
Contact Binaries

Stellar coronae in binary star systems offer both a puzzle and an opportunity. We might expect that large magnetic loop structures on close binaries, such as RS CVn systems and contact binaries, would show evidence for interactions between the stars. While some radio studies support this scenario, there is surprisingly little evidence from EUV and X-ray observations for differences between binary and single star systems. Meanwhile, the binary systems offer observational opportunities through rotational modulation and eclipses of flaring and non-flaring regions. Localizing the sources of coronal emission is key to making the magnetic connection to the underlying photosphere. We discuss the structure of stellar coronae from the perspective of studies of binary systems.

scholarly journals The Sun in time: age, rotation, and magnetic activity of the Sun and solar-type stars and effects on hosted planets

2008 ◽  
Vol 4 (S258) ◽  
pp. 395-408 ◽  
Author(s):  
Edward F. Guinan ◽  
Scott G. Engle
Keyword(s):  
Main Sequence ◽  
High Energy ◽  
Magnetic Activity ◽  
The Sun ◽  
X Ray ◽  
Multi Wavelength ◽  
Solar Type ◽  
Rotation Age ◽  
Energy Emissions ◽  
Solar Analogs

AbstractMulti-wavelength studies of solar analogs (G0–5 V stars) with ages from ~50 Myr to 9 Gyr have been carried out as part of the “Sun in Time” program for nearly 20 yrs. From these studies it is inferred that the young (ZAMS) Sun was rotating more than 10× faster than today. As a consequence, young solar-type stars and the early Sun have vigorous magnetohydrodynamic (MHD) dynamos and correspondingly strong coronal X-ray and transition region/chromospheric FUV–UV emissions (up to several hundred times stronger than the present Sun). Also, rotational modulated, low amplitude light variations of young solar analogs indicate the presence of large starspot regions covering ~5–30% of their surfaces. To ensure continuity and homogeneity for this program, we use a restricted sample of G0–5 V stars with masses, radii, Teff, and internal structure (i.e. outer convective zones) closely matching those of the Sun. From these analogs we have determined reliable rotation-age-activity relations and X-ray–UV (XUV) spectral irradiances for the Sun (or any solar-type star) over time. These XUV irradiance measures serve as input data for investigating the photo-ionization and photo-chemical effects of the young, active Sun on the paleo-planetary atmospheres and environments of solar system planets. These measures are also important to study the effects of these high energy emissions on the numerous exoplanets hosted by solar-type stars of different ages. Recently we have extended the study to include lower mass, main-sequence (dwarf) dK and dM stars to determine relationships among their rotation spin-down rates and coronal and chromospheric emissions as a function of mass and age. From rotation-age-activity relations we can determine reliable ages for main-sequence G, K, M field stars and, subsequently, their hosted planets. Also inferred are the present and the past XUV irradiance and plasma flux exposures that these planets have endured and the suitability of the hosted planets to develop and sustain life.

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