scholarly journals Spectroscopy of Southern Active Stars

1991 ◽  
Vol 130 ◽  
pp. 492-493
Author(s):  
O. Vilhu ◽  
B. Gustafsson ◽  
F.M. Walter
Keyword(s):  
Main Sequence ◽  
Physical Similarity ◽  
Rotation Periods ◽  
Short Rotation ◽  
Cool Stars ◽  
Active Stars ◽  
Active Cool

AbstractHα and HeI D3 lines were observed in five active cool stars over their relatively short rotation periods. Three of the stars (AB Dor, Rstl37B, HD82558) are in the pre main sequence phase (as deduced from their lithium abundances), while two are post main sequence FK Comae type stars (HD32918 and BD-223467) (Vilhu et al., 1991). We present the results of these observations and point out the physical similarity of the Hα and He D3 transitions (Fig.1).

scholarly journals Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

2018 ◽  
Vol 618 ◽  
pp. A48 ◽  
Author(s):  
M. Mittag ◽  
J. H. M. M. Schmitt ◽  
K.-P. Schröder
Keyword(s):  
Main Sequence ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Turnover Time ◽  
Rossby Number ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Rotation Periods ◽  
Period Distribution ◽  
Cool Stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R+HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

The Variation of Lithium Equivalent Width in Active Cool Stars

1986 ◽  
Vol 6 (4) ◽  
pp. 500-503 ◽  
Author(s):  
R. D. Robinson ◽  
K. Thompson ◽  
J. L. Innis
Keyword(s):  
Equivalent Width ◽  
Rotating Stars ◽  
Rotational Modulation ◽  
Best Estimate ◽  
Cool Stars ◽  
Active Stars ◽  
Fast Rotating ◽  
Active Cool

AbstractIn an attempt to determine whether the lithium Une at 6707 Å cornes mainly from the quiet photosphere or from the cooler spots on spotted active stars we have measured Une équivalent widths due to Li and also due to Cal at 6717 Å at various rotation phases on three fast rotating stars, AB Doradus, PZ Telescopium and V 1005 Orionis. The results for AB Dor and PZ Tel show there might be some small rotational modulation which can be linked to the best estimate of the spot position from the available photometry. This moduation is less than that expected on the basis of Giampapa’s work on sunspot spectra. The Li and Cal line equivalent widths were found to anticorrelate.

scholarly journals Gyrochronology and its usage for main sequence field star ages

2008 ◽  
Vol 4 (S258) ◽  
pp. 345-356
Author(s):  
Sydney A. Barnes
Keyword(s):  
Main Sequence ◽  
Rotation Periods ◽  
Field Star ◽  
Stellar Ages ◽  
Cool Stars ◽  
Star Ages

AbstractThe construction of all age indicators consists of certain basic steps which lead to the identification of the properties desirable for stellar age indicators. Prior age indicators for main sequence field stars possess only some of these properties. The measured rotation periods of cool stars are particularly useful in this respect because they have well-defined dependencies that allow stellar ages to be determined with ~20% errors. This method, called gyrochronology, is explained informally in this talk, shown to have the desired properties, compared to prior methods, and used to derive ages for samples of main sequence field stars.

scholarly journals Poplar breeding for the purpose of biomass production in short rotation periods in Germany: Problems and first findings

1993 ◽  
Vol 69 (6) ◽  
pp. 727-729 ◽  
Author(s):  
Horst Weisgerber
Keyword(s):  
Biomass Production ◽  
Plant Material ◽  
High Yield ◽  
Land Utilization ◽  
Rotation Periods ◽  
Short Rotation ◽  
European North ◽  
Basic Plant ◽  
Poplar Breeding ◽  
First Results

Investigations so far on the suitability of poplars for biomass production in short rotation periods in Germany were conducted almost exclusively with clones conceived for conventional cultivation. To introduce improved plant material, a special breeding program was implemented in Hann. Münden. It takes into consideration the most important criteria of suitability for the new form of land utilization. A report is given on findings so far from the realization of this program. Comprehensive basic plant material is available for European, North American and East Asian species from the sections Aigeiros, Leuce and Tacamahaca. Interspecific crossings are being carried out mainly in order to make the best possible use of combination and heterosis effects. A period of at least 20 years is necessary to develop new cultivars with high yield performance and yield stability. First results are presented on hybrids of species which are site-tolerant, resistant and particularly fast-growing at the juvenile phase. Plans to produce multiclonal varieties, each with 5 to 10 suitable clones with the same growing rhythm are in place. Special cultivation recommendations advocate clonal mixtures for each of the most important site units and for different rotation periods of from 2 to 5, 6 to 10 and more than 10 years. Key words: poplar, breeding, biomass, short rotation

scholarly journals Eclipsing spotted giant star with K2 and historical photometry

2018 ◽  
Vol 620 ◽  
pp. A189 ◽  
Author(s):  
K. Oláh ◽  
S. Rappaport ◽  
T. Borkovits ◽  
T. Jacobs ◽  
D. Latham ◽  
...  
Keyword(s):  
Binary System ◽  
Main Sequence ◽  
Rapid Evolution ◽  
Magnetic Activity ◽  
Primary Component ◽  
Physical Parameters ◽  
Giant Star ◽  
Eclipsing Binary ◽  
Giant Stars ◽  
Active Stars

Context. Stars can maintain their observable magnetic activity from the pre-main sequence (PMS) to the tip of the red giant branch. However, the number of known active giants is much lower than active stars on the main sequence (MS) since the stars spend only about 10% of their MS lifetime on the giant branch. Due to their rapid evolution it is difficult to estimate the stellar parameters of giant stars. A possibility for obtaining more reliable stellar parameters for an active giant arises when it is a member of an eclipsing binary system. Aims. We have discovered EPIC 211759736, an active spotted giant star in an eclipsing binary system during the Kepler K2 Campaign 5. The eclipsing nature allows us to much better constrain the stellar parameters than in most cases of active giant stars. Methods. We have combined the K2 data with archival HATNet, ASAS, and DASCH photometry, new spectroscopic radial velocity measurements, and a set of follow-up ground-based BVRCIC photometric observations, to find the binary system parameters as well as robust spot models for the giant at two different epochs. Results. We determined the physical parameters of both stellar components and provide a description of the rotational and long-term activity of the primary component. The temperatures and luminosities of both components were examined in the context of the Hertzsprung–Russell diagram. We find that both the primary and the secondary components deviate from the evolutionary tracks corresponding to their masses in the sense that the stars appear in the diagram at lower masses than their true masses. Conclusions. We further evaluate the proposition that traditional methods generally result in higher masses for active giants than what is indicated by stellar evolution tracks in the HR diagram. A possible reason for this discrepancy could be a strong magnetic field, since we see greater differences in more active stars.

scholarly journals Updated X-ray view of the Hyades cluster

2020 ◽  
Vol 640 ◽  
pp. A66 ◽  
Author(s):  
S. Freund ◽  
J. Robrade ◽  
P. C. Schneider ◽  
J. H. M. M. Schmitt
Keyword(s):  
Main Sequence ◽  
Distribution Functions ◽  
X Rays ◽  
Stellar Rotation ◽  
Detection Rates ◽  
X Ray ◽  
Rotation Periods ◽  
Hyades Cluster ◽  
Bona Fide ◽  
Tidal Tails

Aims. We revisit the X-ray properties of the main sequence Hyades members and the relation between X-ray emission and stellar rotation. Methods. As an input catalog for Hyades members, we combined three recent Hyades membership lists derived from Gaia DR2 data that include the Hyades core and its tidal tails. We searched for X-ray detections of the main sequence Hyades members in the ROSAT all-sky survey, and pointings from ROSAT, the Chandra X-Ray Observatory, and XMM-Newton. Furthermore, we adopted rotation periods derived from Kepler’s K2 mission and other resources. Results. We find an X-ray detection for 281 of 1066 bona fide main sequence Hyades members and provide statistical upper limits for the undetected sources. The majority of the X-ray detected stars are located in the Hyades core because of its generally smaller distance to the Sun. F- and G-type stars have the highest detection fraction (72%), while K- and M-type dwarfs have lower detection rates (22%). The X-ray luminosities of the detected members range from ∼2 × 1027 erg s−1 for late M-type dwarfs to ∼2 × 1030 erg s−1 for active binaries. The X-ray luminosity distribution functions formally differ for the members in the core and tidal tails, which is likely caused by a larger fraction of field stars in our Hyades tails sample. Compared to previous studies, our sample is slightly fainter in X-rays due to differences in the Hyades membership list used; furthermore, we extend the X-ray luminosity distribution to fainter luminosities. The X-ray activity of F- and G-type stars is well defined at FX/Fbol ≈ 10−5. The fractional X-ray luminosity and its spread increases to later spectral types reaching the saturation limit (FX/Fbol ≈ 10−3) for members later than spectral type M3. Confirming previous results, the X-ray flux varies by less than a factor of three between epochs for the 104 Hyades members with multiple epoch data, significantly less than expected from solar-like activity cycles. Rotation periods are found for 204 Hyades members, with about half of them being detected in X-rays. The activity-rotation relation derived for the coeval Hyades members has properties very similar to those obtained by other authors investigating stars of different ages.

Radial Velocity Variations in Cool Supergiants

1984 ◽  
Vol 88 ◽  
pp. 283-288
Author(s):  
Hugh C. Harris
Keyword(s):  
Radial Velocity ◽  
Main Sequence ◽  
Short Period ◽  
Cool Stars ◽  
Synchronous Rotation ◽  
Red Supergiants ◽  
Velocity Variations ◽  
Orbital Periods ◽  
Low Amplitude ◽  
Good Agreement

AbstractA survey of F, G, and W supergiants has been carried out with the DAO radial velocity spectrometer, an efficient instrument for detecting low-amplitude velocity variations in cool stars. Observations of 78 stars over five seasons show generally good agreement with OORAVEL results for spectroscopie binaries. The majority of supergiants show low-amplitude variability, with amplitudes typically 1 to 2 km s−1. The width of the cross-correlation profile has been measured for 58 supergiants. It reveals 14 stars with unusually broad lines, indicative of rotation velocities of 15 to 35 km s−1. Several have short-period binary companions and may be in synchronous rotation. The other broad-lined stars are apparently single or with long orbital periods; they may be making their first transition from the main sequence to become red supergiants.

scholarly journals The effects of rotation on the lithium depletion of G- and K-dwarfs in Messier 35

2020 ◽  
Vol 500 (1) ◽  
pp. 1158-1177
Author(s):  
R D Jeffries ◽  
R J Jackson ◽  
Qinghui Sun ◽  
Constantine P Deliyannis
Keyword(s):  
Rotation Rate ◽  
Binary Systems ◽  
Main Sequence ◽  
Strong Relationship ◽  
Magnetic Activity ◽  
Rich Cluster ◽  
Stellar Radius ◽  
Rotation Periods ◽  
Probability Zero ◽  
Lithium Depletion

ABSTRACT New fibre spectroscopy and radial velocities from the WIYN telescope are used to measure photospheric lithium in 242 high-probability, zero-age main-sequence F- to K-type members of the rich cluster M35. Combining these with published rotation periods, the connection between lithium depletion and rotation is studied in unprecedented detail. At Teff < 5500 K there is a strong relationship between faster rotation and less Li depletion, although with a dispersion larger than measurement uncertainties. Components of photometrically identified binary systems follow the same relationship. A correlation is also established between faster rotation rate (or smaller Rossby number), decreased Li depletion and larger stellar radius at a given Teff. These results support models where star-spots and interior magnetic fields lead to inflated radii and reduced Li depletion during the pre-main-sequence (PMS) phase for the fastest rotators. However, the data are also consistent with the idea that all stars suffered lower levels of Li depletion than predicted by standard PMS models, perhaps because of deficiencies in those models or because saturated levels of magnetic activity suppress Li depletion equally in PMS stars of similar Teff regardless of rotation rate, and that slower rotators subsequently experience more mixing and post-PMS Li depletion.

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.

Lithium in Cool Stars Detected in EUV Surveys

2000 ◽  
Vol 198 ◽  
pp. 366-367 ◽  
Author(s):  
G. Tagliaferri ◽  
L. Pastori ◽  
G. Cutispoto ◽  
R. Pallavicini
Keyword(s):  
Spectral Line ◽  
Spectral Type ◽  
Rotational Velocity ◽  
Luminosity Class ◽  
Photometric Observations ◽  
Cool Stars ◽  
Active Cool

We selected a sample of active cool stars detected in the EUV band by the ROSAT WFC and performed spectroscopic and photometric observations. We inferred spectral type, luminosity class, distance, binary status, rotational velocity. Here we show the results of the Li abundances determination from the Li I 6707.8 Å spectral line.

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