dM Stars Research Articles

Coronal Heating in dMe and dM Stars: Clues from the X-Ray Surface Fluxes

Coronal Heating in dMe and dM Stars: Clues from the X-Ray Surface Fluxes

On the Role of dK and dM Stars in Piling up the Total Mass of Star Clusters

Although many projects were aimed at identifying nearby red dwarfs, the catalogues may be complete up to a limiting distance of 5 pc only and may be reliable to 10 pc. From recent observations it is evident that their absolute visual brightness (MV) ranges from about 6 to at least 18.5 what means that an average dK0 star is at least 100,000 times brighter than red dwarfs of the latest spectral subclasses. The photon detecting efficiency of our instruments is to be increased that these can measure late dM stars from the same distance from which dK stars can be analysed now. Another solution is offered by the flare-active red dwarfs which may be 1000 times brighter during their flare ups than usually.

Aperture Synthesis 12CO and 13CO Observations of DM Tauri: 350 AU Radius Circumstellar Gas Disk

view Abstract Citations (35) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Aperture Synthesis 12CO and 13CO Observations of DM Tauri: 350 AU Radius Circumstellar Gas Disk Saito, M. ; Kawabe, R. ; Ishiguro, M. ; Miyama, S. M. ; Hayashi, M. ; Handa, T. ; Kitamura, Y. ; Omodaka, T. Abstract We have made aperture synthesis observations of 12CO (1-0), 13CO (1-0), 2.6 mm, and 2.7 mm continuum emissions from a young star DM Tau with the Nobeyama Millimeter Array (NMA). It has been found from our observations that 12CO gas with a size of 5"9 x 4"6 is associated with DM Tau and the systematic velocity gradient of 12CO exists along P.A. = 160°. The observational results suggest that DM Tau is surrounded by the disk structure of molecular gas ("circumstellar gas disk") with a radius of 350 AU and an inclination of 40° and that the kinematics of the disk is consistent with Keplerian rotation around DM Tau with a mass of 0.48 Msun. We have also detected the 13CO, 2.6 mm, and 2.7 mm continuum emissions from the disk around DM Tau. The H2 mass of the gas disk in the outer region (r > 100 AU) is estimated to be 2.3 × 10-3 Msun from the 12CO and 13CO emissions, and that in the inner region (r < 100 AU) is 0.019 Msun from 2.6 mm continuum flux. Although the disk around DM Tau is larger than the solar system, the disk may be a protoplanetary disk from the viewpoint that the mass of the disk is comparable to the minimum mass of the solar nebula (∼0.01 Msun) and the kinematics of the disk is consistent with Keplerian rotation. The radial dependence of H2 gas surface density of the disk is roughly estimated as Σ(r) ∝ r-2.0±0.3 using the outer and the inner masses of the disk, unless the depletion of 13CO occurs in the outer region. This power-law index of 2.0±0.3 is significantly larger than the value of 1.5 adopted in the model of minimum solar nebula (Kyoto model). DM Tau is one of the young stars located in the L1551 star-forming region, including HL Tau, GG Tau, and a protostar, L1551-IRS 5. The systemic velocities of gas disks found around these stars in the different evolutional stages are in the very narrow range (VLSR = 5.7 - 6.4 km s-1), and the major axes of the disks are roughly perpendicular to the direction of the local magnetic field. It is likely that these stars were successively formed in the same parent cloud and that the magnetic field might play an important role in the formation of the star-disk systems. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176398 Bibcode: 1995ApJ...453..384S Keywords: STARS: FORMATION; STARS: PRE-MAIN-SEQUENCE; STARS: CIRCUMSTELLAR MATTER full text sources ADS | data products SIMBAD (21)

Properties of X-ray Flares on Young Stars

AbstractThe interest in the giant flares found on young stars results from the fact that their energetics is vastly different from solar flares. The recent ROSAT observations of young clusters yield a good basis for a systematic study of these flares.We have investigated 18 very deep pointed observations of star forming regions in IC348, Chamaeleon, ρ Ophiuchi, and Orion. To study the age dependence of the flare properties we also investigated the observations of the young clusters IC2391, Pleiades, and Hyades. In this way we have evaluated the X-ray emission of several hundred young stars with ages ranging from about 106 years to 7 ń 108 years. We have found 36 large flares on identified cluster members and have determined the flare properties with a simple flare model.The flares show the same relation between the total X-ray flare energy and the quiescent X-ray luminosity as the dM star flares. The total X-ray flare energies decrease smoothly from the very high values found for very young stars to solar values with increasing cluster age. Our results indicate that the large flares on young stars are similar to scaled-up solar-like activity rather than to accretion processes.

SPOT and chromospheric activity on the RS CVN star DM Ursae Majoris

SPOT and chromospheric activity on the RS CVN star DM Ursae Majoris

Light Echo Detection of Circumstellar Disks around Flaring Stars

Light echoes can be used to detect and characterize disks around flaring stars. Such disks are thought to be a hallmark of planet formation but are very difficult to detect by ordinary means. Dwarf emission-line M stars experience flares with luminosities comparable to their quiescent photospheres on time scales of minutes, less than the light travel time across a disk many astronomical units in extent; they are thus ideal candidates for such a search. Bromley (1992, Publ. Astron. Soc. Pac. 104, 1049-1053) calculated that the detection of Jupiter-sized companions using light echoes requires photometric accuracies better than 1 part in 106. However, a disk consisting of grains or small bodies will scatter a much larger fraction of the light than a planet of similar mass. I estimate the light echo amplitudes from plausible geometries of circumstellar material and present simulated light curves. The light echo amplitudes are typically 1% of the flare and I conclude that such events will be detected best in cases where the flare is eclipsed by the star. An examination of the time scales associated with internal processes in a protoplanetary disks around dM stars indicates that any primordial disks may become undetectable in 104 years and will have completely disappeared by 108 years, the estimated age of dMe stars in the solar neighborhood. However, searches for light echoes might constrain the amount of material continuing to fall into these young stellar systems in the form of comet-like objects.

ROSAT X-ray luminosity functions of the Hyades dK and dM stars

A programme of long-duration (∼20-40 ks), ROSAT PSPC pointed observations of the Hyades open star cluster has been conducted. So far, 11 fields have been analysed, yielding, for selected regions (total area ∼3.5 deg 2 ), a sensitivity improvement over the PSPC all-sky survey of more than a factor of 5, and providing high-quality spectra and time series for the brighter sources. Approximately 75 per cent of the Hyads within the fields are detected in X-rays, including 23 out of 32 dM stars and 13 out of 17 dK stars; this allows us to construct their X-ray luminosity functions (XLFs) down to L X ∼5×10 27 erg s −1 (0.1-2.4 keV). All four dMs with M V >13 in our sample have been detected

PG 0308 + 096 and PG 1026 + 002 - Two new short period binary stars resulting from common-envelope evolution

Ultraviolet spectroscopy, optical spectroscopy, and spectrophotometry have been used to study the excess UV stars PG 0308 + 096 and PG 1026 + 002. Both objects are short-period binary systems, each containing a DA white dwarf star and a dM star. Orbital periods of approximately 0.284 day for PG 0308 + 096, and aproximately 0.597 day for PG 1026, have been found by spectroscopic analysis of the H-alpha emission line. Ly-alpha and Balmer line profile fitting were used to estimate the mass of white dwarf stars; mass estimates for the dM stars are based on their spectral types. The orbital inclinations are derived from these masses, the periods, and amplitudes of the H-alpha radial velocity curves. The equivalent width of the H-alpha emission line, in each binary system, varies with the orbital phase in such a manner as to imply that it arises, in large part at least, from the hemisphere of the M star that faces the white dwarf star.

Modeling the stellar contribution to the Galactic component of the diffuse soft X-ray background. I - Background fluxes and number counts

A combination of a stellar Galaxy model based on optical data, stellar X-ray luminosity functions derived from the full Einstein base, and a model for X-ray absorption derived from hydrogen column densities are used to estimate the contribution to the diffuse soft X-ray background flux from the main-sequence A, F, G, K, and M stars, and RS CVn stars, at various energies ranging from 0.1 to about 5 keV. Previous discrepancies between earlier estimates of the stellar contribution to the diffuse soft X-ray background are resolved; this stellar contribution to the diffuse soft X-ray background is found to be under 3 percent for photon energies less than energy I and J bands (about 0.8-2 keV), at a threshold sensitivity for point source detection about 10 exp -10 ergs/s sq cm. At low latitudes, stellar contribution estimates are less than 3 percent below 0.3 keV, 7-40 percent in the medium-energy bands, and 27-70 percent in the I and J bands. It is shown that while dM stars are the major contributors to the diffuse stellar flux, other stellar types contribute as much as 40 percent of this flux at the higher energies in the passband studied.

H-alpha and broadband photometric monitoring of ten M dwarfs - Flares and spots

A set of ten dM3/dM4 stars with a wide range of H-{alpha} equivalent widths, W(H-{alpha}), including both dM and dMe stars, has been monitored in U, V, and H-{alpha}. The time resolution of the data is 3-5 min. A strong correlation is found between the minimum or 'quiescent' value of W(H-{alpha}) for each star and its degree of variability in H-{alpha}. The emission stars in this sample varied by about a factor of 2 in W (H-{alpha}) during flares. A weak correlation also exists between quiescent H-{alpha} and the degree of variability in V. This is caused both by flare contamination of the V bandpass and by the presence of cool spots on at least one of the program stars, Gl 873. Nineteen flare events were detected on the four strongest emission line stars in the sample. To within the crude time resolution of this study, most flares were coincident in U and W(H-{alpha}). However, the largest flare (on Gl 896) clearly showed a lag of at least 20 min between the peak in U and the peak in H-alpha, and the second largest flare showed a similar lag of perhaps 10 min. 17 refs.

Flare activity levels for fully convective red dwarfs

Current stellar models predict fully convective structures for masses less than about 0.3 Mo. We have monitored low luminosity dMe and dM stars for flare activity with the 2.1 m telescope at McDonald Observatory. The dMe stars produced 2-5 detectable U-filter flares per hour while none were seen in the dM stars. Typically, 15-30% of the U-filter flux received from the dMe stars is due to flares. The flare activity level of dMe stars (on an absolute scale) decreases as one approaches the lower end of the main sequence. Non-linear frequency distributions of flare energy are seen in some binary flare stars, suggesting that both stars contribute to the ensemble of flares observed from the binary. Upper limits for the dM stars suggest activity levels about one thousand times smaller than in the dMe stars.

The CA II resonance lines in M dwarf stars without H-alpha emission

Spectra of the Ca II H and K lines in a sample of 31 M dwarf stars without H-alpha emission are used to calculate chromospheric K line radiative losses, F(k), and to study the joint response of Ca II K and H-alpha to chromospheric heating in dwarf M stars. It is suggested that the poor correlation found in the equivalent width - log F(K) diagram may be due either to radial segregation of the H-alpha and K line forming regions or to lateral inhomogeneities in the chromospheres. The results confirm the existence of dM stars with weak H-alpha absorption and K line emission only slightly weaker than that of the dMe stars, and show that dM stars with weak H-alpha but kinematics and metallicities representative of the young disk population belong to a class characterized by a comparatively high degree of chromospheric activity.

BOLOMETRIC LUMINOSITIES AND COLORS FOR K AND M DWARFS AND THE SUBLUMINOUS STARS OF THE HALO

ABSTRACT The H-R diagrams of dM, sdK, and sdM proper-motion stars are examined. A method for integrating energy distributions using discrete weights is proposed. The bolometric corrections are assessed at various wavelengths and a method for obtaining luminosities even if a star lacks IR data is presented. The color-luminosity diagrams suggest that high-velocity, low-metallicity stars of the halo are subluminous. It is found that the apparent cutoff in the halo is a bolometric magnitude of about 12 m.

Far-infrared properties of flare stars and dM stars

Results are reported from a search of the IRAS data base for flare stars and for a control sample of dM stars. At 12 microns, 70-80 percent of both samples have been detected. The K-12 colors of flare stars are significantly different from those of dM stars: for a given K magnitude, a flare star is about 70 percent brighter at 12 microns than a dM star. At 100 microns, 27 percent of the flare stars which are sources at 12 microns have been detected, while none of the comparable dM stars has been detected. Implications for microflaring are discussed.

Optical Chromospheric Spectral Lines in K and M Dwarf Stars

AbstractObservations are reported of the Ca II resonance lines and II α in dK and dM stars, made with high S/N ratio and high spectral resolution. Ca II emission is found in all stars observed, and those having weak Ca II exhibit marked Hα absorption. It is found that the strengths of the two kinds of chromospheric, lines are not tightly correlated, an effect which can be shown to be independent of the effective temperature of the stars. The result implies that a one-parameter description (e.g. heating rate) of the chromospheres is not viable. While lateral inhomogeneities are likely to be an important second parameter, we also suggest that the Hα line may be formed in a region considerable higher that in which the Ca II lines are formed.

VARIABILITY OF CHROMOSPHERIC LINES IN LATE-TYPE DWARF STARS

ABSTRACT The results of a high-resolution spectroscopic synoptic program of observations of key chromospheric diagnostics in a sample of dwarf stars are reported. Low-level variability in the strength of the chromospheric H-alpha absorption line is found. The nature of the variability implies that the operative chromospheric heating mechanism is partially characterized by a transient component. This fact, combined with the detectability of the low-level variability, suggests that magnetic field-related, nonradiative atmospheric heating is widespread in the dM stars. High-amplitude variability in the H-alpha emission strength of the active dMe star BY Dra was also observed. Synoptic observations of the He I 5876 A absorption line in three solar-type stars revealed low-level variability in the strength of this feature.

OBSERVATIONS OF GLIESE 890 AND OTHER POSSIBLY RAPIDLY ROTATING FIELD M DWARFS

ABSTRACT BV photometry of the rapidly-rotating M dwarf Gliese 890 (dM2e) obtained in 1983-84 is presented. The long-term photometric behavior of the star is examined, and evidence is found for the presence of persistent polar active regions, as has been proposed by Young at al. (1987). In an attempt to detect additional rapidly-rotating field M dwarfs similar to Gliese 890, four dM stars suspected by Cowley and Hartwick (1982) to show broadened absorption lines are observed. CCD spectra with 1-A resolution fail to show any evidence for line broadening. The H-alpha line is in absorption in all four stars.

H-alpha photometry of dwarf K and M stars

view Abstract Citations (37) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hα Photometry of Dwarf K and M Stars Herbst, William ; Layden, Andrew C. Abstract Hα photometry of 188 dM and dK stars has been obtained with the Perkin telescope at Van Vleck Observatory. The photometric index, N-W, has been calibrated using 56 stars with equivalent widths determined spectroscopically by Stauffer and Hartmann (1986). An external standard error of 0.08 Å is found for the equivalent width (EW) measurements of non-emission stars; it may be higher for those with emission lines. The distribution of stars in the N-W, R-I plane is discussed. Most lie along a single "main sequence" of absorption which slopes up towards 0 EW with increasing R-I. Eighteen dKe or dMe stars are found, all clearly separated from the absorption sequence. The absorption EWs are large compared with expectation based on photospheric models, indicating either that the models are in error or that chromospheres are present in essentially all late-type dwarfs. Three stars (Gl 182, Gl 873AB, and Gl 890) were found to vary significantly in N-W. All are emission stars, and the variability can probably be attributed to flares. Publication: The Astronomical Journal Pub Date: July 1987 DOI: 10.1086/114457 Bibcode: 1987AJ.....94..150H Keywords: Dwarf Stars; H Alpha Line; K Stars; M Stars; Stellar Spectrophotometry; Variable Stars; Emission Spectra; Hertzsprung-Russell Diagram; Main Sequence Stars; Stellar Magnitude; Astrophysics; STARS: ABUNDANCES; STARS: SPECTRAL CLASSIFICATION full text sources ADS | data products SIMBAD (171)

Atmospheric activity in red dwarf stars

Radiation losses from quiescent and flaring regions of red dwarf atmospheres are compared and found to be equally important. The flare emission on all wavelengths approaches 1% of the stellar bolometric luminosity for active dMe stars, but is 100 times less for some dM stars. The discriminating parameter is flare frequency, which may vary with stellar mass, age, rotation rate or other quantities. Radiation losses from chromospheric emission lines are larger than losses from transition region lines. H I is the most important element in late dMe stars, while Ca II dominates the chromospheric radiation loss from early dMe and dKe stars. The corona is less significant in dKe stars, but dominates over chromospheric radiation losses in dMe stars. A sharp drop in coronal importance is seen near dM5e, where stars become fully convective. All forms of radiation losses have a common cause, probably to be found in the convection zone. The magnetic field may be the instrument that brings mechanical energy into the outer atmosphere, where it is radiated into space.

The orbit and properties of the HD 149162 system

view Abstract Citations (4) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Orbit and Properties of the HD 149162 System Johnson, Hugh M. ; Mayor, M. Abstract HD 149162 is classified photometrically single and SB1 except for H-alpha emission that departs widely from the radial velocity of the K0 dwarf. Ca II H and K and Mg II h and k also show emission components at low dispersion. The system is X-ray luminous although ostensibly old disk and a slow K dwarf rotator. The orbit and other observations lead to a secondary that is composed of a dMe star and a cool white dwarf or a pair of dM and dMe stars. Further observations of the emission lines should ultimately define the triple system. Publication: The Astrophysical Journal Pub Date: November 1986 DOI: 10.1086/164688 Bibcode: 1986ApJ...310..354J Keywords: Binary Stars; Orbit Calculation; Stellar Motions; Stellar Orbits; K Stars; Late Stars; Radial Velocity; Stellar Evolution; Ultraviolet Astronomy; White Dwarf Stars; Astrophysics; STARS: BINARIES; STARS: INDIVIDUAL HENRY DRAPER NUMBER: HD 149162; STARS: LATE-TYPE; ULTRAVIOLET: SPECTRA full text sources ADS | data products SIMBAD (1) MAST (1) INES (1)