scholarly journals Statistics of Stellar Variability in Kepler Data with ARC Systematics Removal

2011 ◽  
Vol 7 (S285) ◽  
pp. 364-365
Author(s):  
Amy McQuillan ◽  
Suzanne Aigrain ◽  
Stephen Roberts
Keyword(s):  
Main Sequence ◽  
Active Regions ◽  
Life Time ◽  
Characteristic Size ◽  
Proper Motions ◽  
Main Sequence Stars ◽  
Periodic Behaviour ◽  
Tentative Evidence ◽  
M Stars ◽  
F Stars

AbstractWe investigate the variability properties of main-sequence stars in the first month of Kepler data, using a new astrophysically robust systematics correction. We find that 36% appear more variable than the Sun, and confirm the trend of increasing variability with decreasing effective temperature. We define low- and high-variability samples, with a cut at twice the level of the active Sun, and compare properties of the stars belonging to each sample. We find tentative evidence that the more active stars have lower proper motions. The frequency content of the variability shows clear evidence for periodic or quasi-periodic behaviour in 16% of stars, and highlights significant differences in the nature of variability between spectral types. Most A and F stars have short periods (< 2 days) and highly sinusoidal variability, suggestive of pulsations, whilst G, K and M stars tend to have longer periods (> 5 days, with a trend towards longer periods at later spectral types) and show a mixture of periodic and stochastic variability, indicative of activity. Finally, we use autoregressive models to characterise the stochastic component of the variability, and show that its typical amplitude and time-scale increase towards later spectral types, which we interpret as an increase in the characteristic size and life-time of active regions. Full details will be published shortly.

scholarly journals Medium Resolution Spectroscopy at 1-2 Microns

1979 ◽  
Vol 47 ◽  
pp. 239-246
Author(s):  
J. R. Mould
Keyword(s):  
Fourier Transform ◽  
Main Sequence ◽  
Classification Criteria ◽  
Fourier Transform Spectrometer ◽  
Main Sequence Stars ◽  
Composite Spectra ◽  
Medium Resolution ◽  
M Stars

AbstractThe need for establishing classification criteria at long wavelengths is stressed. The usefulness of doing this is illustrated with a discussion of the composite spectra of FU Orionis stars. Spectra of these pre-main-sequence stars from 1.5-2.5μ were obtained with a Fourier Transform Spectrometer. Luminosity criteria in the l-2μ range are also discussed with application to M stars.

scholarly journals NGTS clusters survey – II. White-light flares from the youngest stars in Orion

2020 ◽  
Vol 497 (1) ◽  
pp. 809-817 ◽  
Author(s):  
James A G Jackman ◽  
Peter J Wheatley ◽  
Jack S Acton ◽  
David R Anderson ◽  
Claudia Belardi ◽  
...  
Keyword(s):  
White Light ◽  
Main Sequence ◽  
High Energy ◽  
Occurrence Rate ◽  
Next Generation ◽  
Main Sequence Stars ◽  
M Stars ◽  
Flaring Stars

ABSTRACT We present the detection of high-energy white-light flares from pre-main-sequence stars associated with the Orion Complex, observed as part of the Next Generation Transit Survey (NGTS). With energies up to 5.2 × 1035 erg these flares are some of the most energetic white-light flare events seen to date. We have used the NGTS observations of flaring and non-flaring stars to measure the average flare occurrence rate for 4 Myr M0–M3 stars. We have also combined our results with those from previous studies to predict average rates for flares above 1 × 1035 erg for early M stars in nearby young associations.

scholarly journals Discovery of Three Very Low Mass Binary Systems: An Adaptive Optics Survey of M6.0–M7.5 Stars

2003 ◽  
Vol 211 ◽  
pp. 257-260
Author(s):  
Nick Siegler ◽  
Laird M. Close ◽  
Eric E. Mamajek ◽  
Melanie Freed
Keyword(s):  
Adaptive Optics ◽  
Binary Systems ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Solar Mass ◽  
Low Mass ◽  
M Stars ◽  
Adaptive Optics System

We have used the adaptive optics system Hōkūpa'a at Gemini North to search for companions from a flux-limited (Ks > 12) survey of 30 nearby M6.0–M7.5 dwarfs. Our observations, which are sensitive to companions with separations > 0.1″ (~ 2.8 AU), detect 3 new binary systems. This implies an overall binary fraction of 9±4% for M6.0–M7.5 binaries. This binary frequency is somewhat less than the 19±7% measured for late M stars and ~ 20% for L stars, but is still statistically consistent. However, the result is significantly lower than the binary fractions observed amongst solar mass main sequence stars (~60%) and early M stars (~35%).

scholarly journals The Frequency of Faint M Giant Stars at High Galactic Latitudes

1977 ◽  
Vol 4 (2) ◽  
pp. 35-36 ◽  
Author(s):  
N. Sanduleak
Keyword(s):  
Main Sequence ◽  
Galactic Plane ◽  
Absolute Magnitude ◽  
Main Sequence Stars ◽  
Giant Stars ◽  
The North ◽  
Objective Prism ◽  
M Stars ◽  
Low Dispersion ◽  
North Galactic

Based on the observations of M giant stars in the north galactic polar objective-prism survey of Upgren (1960) and the data summarized by Blanco (1965) the overall space density of all M-type giants as a function of distance from the galactic plane at the position of the sun can be approximated by,where z is in kpc and ρ(z) is the number of stars per 106 pc3. This relationship is derived from the observed fall-off in space densities up to a distance of about 2 kpc.The question arises as to the validity of extrapolation equation (1) to larger z distances so as to predict the number of faint M giants expected per unit area near the galactic poles. Adopting for the M giants a mean visual absolute magnitude of −1.0 (Blanco 1965), one finds that equation (1) predicts that less than one giant fainter than V~12 should be expected in a region of 200 square degrees. This expectation formed the hypothesis of a thesis study (Sanduleak 1965) in which it was assumed that the very faint M stars detected in a deep, infrared objective-prism survey at the NGP were main-sequence stars, since this could not be ascertained spectroscopically on the very low-dispersion plates used.

scholarly journals Polarization of Young Shell Variables

1971 ◽  
Vol 15 ◽  
pp. 40-44
Author(s):  
M. Breger
Keyword(s):  
Small Particle ◽  
Main Sequence ◽  
Tauri Star ◽  
Particle Sizes ◽  
Main Sequence Stars ◽  
T Tauri ◽  
Magnitude Diagram ◽  
The Mean ◽  
F Stars

AbstractStellar polarization between 3000 and 7500 Å has been measured in NGC 2264. Out of 37 stars (including many variables) 4 are definitely polarized, while 3 others show weak polarization. The mean (interstellar) cluster polarization is found to be very small. The highest polarization (4,6 %) is found in a T Tauri star. W 90, a variable below the main sequence, has the highest polarization found among the A and F stars (2, 6%). With the exception of W46, polarization peaks in the blue implying small particle sizes. A good correlation is found between the size of polarization and (other) shell characteristics. These measurements confirm the recently proposed shells in some pre-main sequence stars which appear to be too faint for their color, but fail to detect any new stars with extensive shells. This suggests that some of the scatter in the color-magnitude diagram amongst the A and F stars is real because of an age spread between 1 and 3 X 106 years.

scholarly journals Fast-rotating giant stars behind the Coma Berenices star cluster

2020 ◽  
Vol 497 (3) ◽  
pp. 2562-2568
Author(s):  
Estefanía Casal ◽  
Matilde Fernández ◽  
Emilio J Alfaro ◽  
Víctor Casanova ◽  
Ángel Tobaruela
Keyword(s):  
Main Sequence ◽  
Star Cluster ◽  
Stellar Clusters ◽  
Main Sequence Stars ◽  
Cluster Membership ◽  
Giant Stars ◽  
Data Release ◽  
Magnitude Diagram ◽  
M Stars ◽  
Fast Rotating

ABSTRACT In the frame of a study of the empirical isochrones of young stellar clusters, we have carried out BVIc Johnson–Cousins photometry of a sample of K and M stars of the Coma Berenices star cluster. All these stars have known rotational periods. Our main goal is to get a valuable reference on the colour–magnitude diagram, Mv versus B − V, for stars with ages within 400–800 Myr. For this purpose, we obtained BVIc photometry with an average upper limit for the precision of about 0.025 mag and used parallaxes from the Gaia Data Release 2. We found that one-third of our sample is located well above the cluster main sequence and these stars are confirmed as background giants by their radial velocities in the Gaia Data Release 2. This misclassification shows that giants with short-surface rotational periods can mimic main-sequence stars if they are located at the appropriate distance. We recommend caution when using rotational periods in order to determine cluster membership. Besides, the gyrochronology technique should be used only when the luminosity class of the stars is well known. Finally, our cleared sample supports an age of ∼600 Myr for Coma Berenices, rather than an age of ∼800 Myr.

Kinematics of main-sequence stars from the Gaia DR2 and PMA proper motions

2020 ◽  
Vol 494 (1) ◽  
pp. 1430-1447
Author(s):  
Anna B Velichko ◽  
P N Fedorov ◽  
V S Akhmetov
Keyword(s):  
Velocity Field ◽  
Main Sequence ◽  
Vertical Gradient ◽  
Model Parameters ◽  
Separate Analysis ◽  
Proper Motions ◽  
Kinematic Parameters ◽  
Main Sequence Stars ◽  
Stellar Velocity ◽  
Gaia Dr2

ABSTRACT We aim at analysis of kinematics of main-sequence stars from the GaiaDR2 and PMA catalogues as well as comparison of kinematic parameters derived from their proper motions. We decompose the stellar velocity field on to a set of vector spherical harmonics (VSH), and, using the relations between the decomposition coefficients and the Ogorodnikov–Milne (O–M) model parameters, calculate the latter. The method of VSH allowed to detect all systematic constituents present in the GaiaDR2 and PMA stellar velocity fields. We notice incompleteness of the O–M model, discuss the kinematic parameters derived within its framework, as well as significant decomposition coefficients that do not have analogues within this model. For separate analysis of the kinematic parameters in the Northern and Southern Galactic hemispheres, we apply the decomposition on to a set of zonal VSH (ZVSH). Modelling the stellar velocity field allowed to confirm the causes of some significant beyond-the-model harmonics. Based on stellar proper motions from the Gaia DR2 and PMA catalogues, we confirm the previous conclusion that the values of ω1 and $M^+_{23}$ O–M model parameters derived in the Northern and Southern Galactic hemispheres have opposite signs. This fact takes place due to a vertical gradient of the linear rotation velocity about the Galactic centre. We estimate influence of the vertical gradient on the value of solar velocity component Y⊙. We confirm that the beyond-the-model coefficients t211 and s310 are caused by the vertical gradient as well.

scholarly journals Photometric detection of internal gravity waves in upper main-sequence stars

2019 ◽  
Vol 621 ◽  
pp. A135 ◽  
Author(s):  
D. M. Bowman ◽  
C. Aerts ◽  
C. Johnston ◽  
M. G. Pedersen ◽  
T. M. Rogers ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Gravity Waves ◽  
Main Sequence ◽  
Low Frequency ◽  
Internal Gravity Waves ◽  
Main Sequence Stars ◽  
Photometric Detection ◽  
F Stars ◽  
Low Frequency Power ◽  
Frequency Power

Context. Main sequence stars with a convective core are predicted to stochastically excite internal gravity waves (IGWs), which effectively transport angular momentum throughout the stellar interior and explain the observed near-uniform interior rotation rates of intermediate-mass stars. However, there are few detections of IGWs, and fewer still made using photometry, with more detections needed to constrain numerical simulations. Aims. We aim to formalise the detection and characterisation of IGWs in photometric observations of stars born with convective cores (M ≳ 1.5 M⊙) and parameterise the low-frequency power excess caused by IGWs. Methods. Using the most recent CoRoT light curves for a sample of O, B, A and F stars, we parameterised the morphology of the flux contribution of IGWs in Fourier space using an MCMC numerical scheme within a Bayesian framework. We compared this to predictions from IGW numerical simulations and investigated how the observed morphology changes as a function of stellar parameters. Results. We demonstrate that a common morphology for the low-frequency power excess is observed in early-type stars observed by CoRoT. Our study shows that a background frequency-dependent source of astrophysical signal is common, which we interpret as IGWs. We provide constraints on the amplitudes of IGWs and the shape of their detected frequency spectrum across a range of mass, which is the first ensemble study of stochastic variability in such a diverse sample of stars. Conclusions. The evidence of a low-frequency power excess across a wide mass range supports the interpretation of IGWs in photometry of O, B, A and F stars. We also discuss the prospects of observing hundreds of massive stars with the Transiting Exoplanet Survey Satellite (TESS) in the near future.

scholarly journals Stellar Spectroscopy at 1.1 μ

1974 ◽  
Vol 3 ◽  
pp. 357-359
Author(s):  
H. Zirin
Keyword(s):  
Main Sequence ◽  
Active Regions ◽  
Progress Report ◽  
The Sun ◽  
Image Converter ◽  
Main Sequence Stars ◽  
Stellar Spectroscopy ◽  
Cyclic Variations ◽  
Cold Box ◽  
Variable Quality

This is a progress report on the program of stellar spectroscopy at 1.1 μ reported on by Vaughan and Zirin (1968) and by Zirin (1971). About 450 plates of about 200 stars have been obtained, using a magnetically focused RCA image converter with the 144” and 72” Palomar coudé cameras, giving dispersion of 8 and 17 Å mm-1 respectively. Most of the plates were taken at the latter dispersion and cover about 300 Å. In good seeing a star with J = 3 is obtained in about 90 min. A tube with a fibre optic backplate is now in use. Cooling such a tube is difficult because the cathode is held at high voltage. By completely insulating the cold box, we may use dry ice cooling, and 2 h exposures, reaching J = 5, may be made with this ITT tube.Our principal interest in this region of the spectrum is the He I 10830 line, but there are other features of interest such as Paschen y and a series of CN bands, λ 10830 is of particular interest because it is produced at relatively high temperatures and thus its presence in stars later than B5 can only be explained by the existence of a high temperature corona or chromosphere or of a strong ultraviolet source, which amounts to the same thing. Vaughan and Zirin found the presence of such chromospheres in a number of stars, and I have pursued the matter since then to determine the existence of chromospheric variability. The 10830 absorption in the Sun is very small from the normal chromosphere and is principally due to the presence of active regions. Emission is only found in large flares. Thus cyclic variations in Sun-like stars could be attributed to sunspot cycles. Unfortunately, main sequence stars are near the limit of our observations. Although definite 10830 absorption has been found in such main sequence stars as 61 Cyg A and B, 70 Oph br, K Cet and ε Eri, the absorption is just barely detectable and variation from plate to plate could be explained by variable quality.

Main Sequence Abundances: Observational Aspects

1988 ◽  
Vol 108 ◽  
pp. 2-2
Author(s):  
Jun Jugaku
Keyword(s):  
Chemical Composition ◽  
Effective Temperature ◽  
Main Sequence ◽  
Theoretical Models ◽  
Current Data ◽  
Main Sequence Stars ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Chemically Peculiar Stars ◽  
F Stars

AbstractAlthough once it was thought that main-sequence stars are remarkably homogeneous with respect to their chemical composition, the upper main-sequence stars (30000 > Te > 7000) show a variaety of chemically peculiar stars besides the so-called normal stars. Those include the Am, Ap, λ Bootis, He-deficient, and He-rich stars. This review summarizes the current data, which are necessary to construct and test the theoretical models of these stars. In the second half of the review we concentrate on Li. In the lower main-sequecnce stars abundances of Li have been determined in hundreds of stars. Some of the remarkable results are: (1) A uniform upper abundance value irrespective of stellar effective temperature, (2) abundance gap in the F stars of the Hyades, and (3) increasing depletion with smaller stellar mass for the Hyades.

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