dK Stars Research Articles

Observation and modelling of main sequence star chromospheres

Aims. We report on high resolution observations of chromospheric lines for M and K dwarfs. Methods. We observed up to 13 spectral lines per star: the Hα ,H β ,H � ,P a � ,P a 8, CaII H, CaII IR triplet, CaI 6572A, NaI D1/D2 and HeI D3 lines. We observed two dMe stars, one dM(e) star, 5 dM stars and 3 dK stars. Results. We observed a self-reversal in the emission core of the CaII H line for the brightest stars only, indicating a rather optically thick region of formation. We present original spectra of the NaI doublet and the CaII IR triplet for active dMe stars and less active dM stars. Core emission is detected in the Sodium lines and the CaII IR triplet lines for the most active M dwarf AU Mic. We investigate the difference spectra between active dMe stars and dM stars and show that these provide interesting new constraints for the NLTE-radiation transfer modelling of the chromospheres. In our sample, emission Hα profiles have a rather homogeneous FWHM of about 1.5 A. This, according to our previous modelling, can be interpreted as the signature of a rather constant temperature break in the chromosphere. We found that one of our targets (MCC 332) is a binary with a faint but active Hα emission component. – –

Mg II h+k emission lines as stellar activity indicators of main sequence F-K stars

The main purpose of this study is to use the IUE spectra in the analysis of magnetic activity of main sequence F-K stars. Combining IUE observations of MgII and optical spectroscopy of Ca II, the registry of ctivity of stars can be extended in time. We retrieved all the high-resolution spectra of F, G, and K main sequence stars observed by IUE (i.e. 1623 spectra of 259 F to K dwarf stars). We obtained the continuum surface flux near the Mg II h+k lines near 2800 Aand the MgII line-core surface flux from the IUE spectra. We obtained a relation between the mean continuum flux near the MgII lines with the colour $B-V$ of the star. For a set of 117 nearly simultaneous observations of Mg II and Ca II fluxes of 21 F5 to K3 main sequence stars, we obtained a colour dependent relation between the Mount Wilson CaII S-index and the MgII emission line-core flux. As an application of this calibration, we computed the Mount Wilson index for all the dF to dK stars which have high resolution IUE spectra. For some of the most frequently observed main sequence stars, we analysed the Mount Wilson index S from the IUE spectra, together with the ones derived from visible spectra. We confirm the cyclic chromospheric activity of epsilon Eri (HD 22049) and beta Hydri (HD 2151), and we find a magnetic cycle in alpha Cen B (HD 128621). Complete abstract in the paper.

Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests

We present an extensive study on the dependence of the convective turnover time (conv) on the stellar metallicity and age for main-sequence stars of masses 0.6{1.6 M. To this aim we have used and compared predictions by stellar models based on the classical Mixing Length Theory and models incorporating a Full Spectrum of Turbulence treatment of subphotospheric convection. We show that the metallicity eect is relevant for dG stars but negligible for dK stars, while stellar age is important when computing the turnover times for red dwarfs younger than logt 8:5 yr. A scatter by up to a factor 3 could be spuriously introduced in the activity vs. Rossby number relationships if such eects are neglected. We have tested our model predictions on the metallicity eect by comparison with the observed median X-ray luminosities for dG and dK stars in the open clusters NGC 2516, the Pleiades, and Blanco 1, having similar age but likely dierent metal contents. We show that our model predictions, taking into account also the dependence of the coronal plasma emissivity on the metal abundance, are in agreement with the observations in all cases except for the dK stars in the metal-rich cluster Blanco 1, where our assumption of a rotation period distribution similar to those of the other cluster need to be conrmed by future observations.

XMM-Newton survey of the low-metallicity open cluster NGC 2516

We present the first results of an XMM-Newton EPIC survey of NGC 2516, a southern low-metallicity open cluster with an age close to the Pleiades. The attained limiting sensitivity is of ~2.4 10-15 erg s-1 cm-2 in the 0.1-4.0 keV bandpass. This has been achieved by summing the data of the MOS and PN cameras of two distinct observations for a total exposure time of ~33 ks and by analyzing the summed data set with the wavelet detection algorithm developed at Osservatorio Astronomico di Palermo (OAPA) that has yielded over 200 X-ray detections. Using data just from a single exposure or from a single camera would have reduced by a factor 2-4 our limiting sensitivity and would have resulted in 25-40% less X-ray detections. To date, 129 detections have as counterparts one or more of the 540 photometrically selected cluster members in the surveyed region, for a total of 147 likely detected members, with unique identification in 112 cases. We derive the X-ray luminosity functions (XLF) of NGC 2516 members of different spectral types and compare them with those of the more metal rich, approximately coeval Pleiades cluster, finding the NGC 2516 photometrically selected dG and dK stars less luminous than the Pleiades. The XLFs of the NGC 2516 and of the Pleiades dM stars are indistinguishable. We compare the XMM-Newton results with those recently obtained with Chandra .

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.

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

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)

The stellar contribution to the galactic soft X-ray background

Log N-log S relations for stars are constructed based on median X-ray luminosities for dF, dG, and dK stars previously reported for the Einstein Observatory/Center for Astrophysics stellar survey and on a detailed X-ray luminosity function derived here for dM stars, and the stellar contribution to the diffuse soft X-ray background is investigated. The principal results are that stars provide approximately 20% of the soft X-ray background in the 0.28-1.0 keV passband and therefore contribute significantly to the soft X-ray background in this energy range (with dM stars constituting the dominant contributing class), and that the stellar contribution to the diffuse X-ray background in the 0.15-0.28 keV passband is less than approximately 3%.