Subluminous Stars

The term white dwarf was originally introduced, some 50 years ago, to describe intrinsically faint A-type stars. Spectroscopic studies and theoretical considerations confirm the suggestion that these are degenerate objects. Photographic colorimetry of faint proper motion stars can readily distinguish the blue objects and in this way several hundred white dwarf candidates have been discovered, mainly by Luyten. Accurate photometry of about 1000 of these stars has led to accurate estimates for the space density of 1.5 = 10−3 pc−3. However, subluminous late-type stars are more difficult to detect among the multitude of main sequence stars and some additional criteria are needed.

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On the Separations of Common Proper Motion Binaries Containing White Dwarfs

International Astronomical Union Colloquium ◽

10.1017/s0252921100100065 ◽

1989 ◽

Vol 114 ◽

pp. 454-457

Author(s):

T.D. Oswalt ◽

E.M. Sion

Keyword(s):

Mass Loss ◽

White Dwarf ◽

Proper Motion ◽

White Dwarfs ◽

Main Sequence ◽

Late Type ◽

Class A ◽

Color Class

Luyten [1,2] and Giclas et al. [3,4] list over 500 known common proper motion binaries (CPMBs) which, on the basis of proper motion and estimated colors, are expected to contain at least one white dwarf (WD) component, usually paired with a late type main sequence (MS) star. Preliminary assessments of the CPMBs suggest that nearly all are physical pairs [5,6]. In this paper we address the issue of whether significant orbital expansion has occurred as a consequence of the post-MS mass loss expected to accompany the formation of the WDs in CPMBs.Though the CPMB sample remains largely unobserved, a spectroscopic survey of over three dozen CPMBs by Oswalt [5] found that nearly all faint components of Luyten and Giclas color class “a-f” and “+1”, respectively, or bluer were a WD. This tendency was also evident in a smaller sample studied by Greenstein [7]. Conversely, nearly all CPMBs having two components of color class “g-k” and “+3” or redder were MS+MS pairs. With the caveat that such criteria discriminate against CPMBs containing cool (but rare) WDs, they nonetheless provide a crude means of obtaining statistically significant samples for the comparison of orbital separations: 209 highly probable WD+MS pairs and 109 MS+MS pairs.

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The Luminosity Function of Late-Type Main-Sequence Stars in the Direction of the North Galactic Cap

Highlights of Astronomy ◽

10.1007/978-94-010-1250-8_8 ◽

1977 ◽

pp. 31-31

Author(s):

Donna Weistrop

Keyword(s):

Luminosity Function ◽

Main Sequence ◽

Late Type ◽

Main Sequence Stars ◽

The North ◽

North Galactic

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Periodic Light Variability in Four Late-Type Pre-Main-Sequence Stars

Astrophysics and Space Science Library - Activity in Red-Dwarf Stars ◽

10.1007/978-94-009-7157-8_71 ◽

1983 ◽

pp. 503-504

Author(s):

F. J. Vrba ◽

A. E. Rydgren ◽

J. T. Schmelz

Keyword(s):

Main Sequence ◽

Late Type ◽

Main Sequence Stars ◽

Light Variability

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Photometry of AM Her Stars – Line and Continuum Emission

International Astronomical Union Colloquium ◽

10.1017/s0252921100076132 ◽

1979 ◽

Vol 53 ◽

pp. 324-328

Author(s):

Paula Szkody

Keyword(s):

Emission Line ◽

White Dwarf ◽

Main Sequence ◽

Close Binary ◽

Line Spectrum ◽

Continuum Emission ◽

Late Type ◽

Emission Line Spectrum ◽

Orbital Periods

The 4 known AM Her stars or polars (AM Her, ANUMa, W Pup, and 2A0311-227) are characterized by large circular polarizations of 10-35%, (Tapia 1977a, b, Krzeminski and Serkowski 1977), an emission line spectrum with strong H and He lines (Crampton and Cowley 1977, Greenstein et al. 1977), complex photometric variations (Szkody 1978, Priedhorsky and Krzeminski 1978, Warner & Nather 1972), long term high and low states and short orbital periods (80-180 min.). Models of these systems envision a close binary containing a magnetic white dwarf primary (B ~ 108G) and late type main sequence secondary transferring material into an accretion funnel over one or both poles of the white dwarf (Stockman et al. 1977, Lamb & Masters 1979, Liebert et al. 1978).

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A preliminary color-magnitude diagram for late-type stars in the solar neighborhood

Symposium - International Astronomical Union ◽

10.1017/s0074180900177502 ◽

1959 ◽

Vol 10 ◽

pp. 39-40

Author(s):

O. C. Wilson

Keyword(s):

Main Sequence ◽

Star Clusters ◽

Solar Neighborhood ◽

Late Type ◽

Main Sequence Stars ◽

Individual Values ◽

Magnitude Diagram ◽

Trigonometric Parallaxes ◽

Nearby Stars ◽

The Mean

Modern photoelectric techniques yield magnitudes and colors of stars with accuracies of the order of a few thousandths and a few hundredths of a magnitude respectively. Hence for star clusters it is possible to derive highly accurate color-magnitude arrays since all of the members of a cluster may be considered to be at the same distance from the observer. It is much more difficult to do this for the nearby stars where all of the objects concerned are at different, and often poorly determined, distances. If one depends upon trigonometric parallaxes, the bulk of the reliable individual values will refer to main sequence stars, and while the mean luminosities of brighter stars are given reasonably well by this method, the scatter introduced into a color-magnitude array by using individual trigonometrically determined luminosities could obscure important features. Somewhat similar objections could be raised against the use of the usual spectroscopic parallaxes which also should be quite good for the main sequence but undoubtedly exhibit appreciable scatter for some, at least, of the brighter stars.

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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.

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Rotation of Pre-Main Sequence Stars from High S/N Spectroscopy

Symposium - International Astronomical Union ◽

10.1017/s0074180900034835 ◽

1988 ◽

Vol 132 ◽

pp. 95-98

Author(s):

J. Bouvier

Keyword(s):

Stellar Evolution ◽

Main Sequence ◽

Spectroscopic Studies ◽

Main Sequence Stars ◽

Rotation Rates ◽

Large Samples ◽

Stellar Formation ◽

Low Mass ◽

Pms Stars ◽

Large Telescopes

Until 1980, only a handful of low-mass, active pre-main sequence (pms) stars had known rotation velocities (vsini) /1/. Since then, increasingly sensitive detectors coupled to large telescopes led to high–resolution (a few 104) spectroscopic studies of these faint stars (mv = 10–13), with S/N ratio of the order of 100. The measurement of vsini for large samples of pms stars that resulted brought new insights on various pressing questions related to stellar formation and early stellar evolution : how do the rotation rates of pms stars compare with those expected from models of stellar formation ? how does the stellar angular momentum change during pms evolution ? is pms activity linked with rotation as would be expected if activity were triggered by magnetic processes ?

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Spectroscopic Analyses of Hot Main Sequence Stars

International Astronomical Union Colloquium ◽

10.1017/s025292110009182x ◽

1986 ◽

Vol 90 ◽

pp. 369-380

Author(s):

Kozo Sadakane

Keyword(s):

Present Status ◽

Main Sequence ◽

Spectroscopic Studies ◽

Uv Spectra ◽

Main Sequence Stars ◽

Visual Region ◽

Spectroscopic Analyses

AbstractSpectroscopic studies of normal O and early B type stars in the visual region are discussed. Present status of UV spectroscopic analyses of hot normal stars is reviewed. Discussions on a few practical problems in analyses of UV spectra are presented.

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