내용
사진
사진 업로드
DSS Images Other Images
관련 글
Metallicities and activities of southern stars
Aims. We present the results from high-resolution spectroscopicmeasurements to determine metallicities and activities of bright starsin the southern hemisphere. Methods: We measured the iron abundances([Fe/H]'s) and chromospheric emission indices (log h{R}'{HK})of 353 solar-type stars with V = 7.5-9.5. [Fe/H] abundances aredetermined using a custom χ2 fitting procedure within alarge grid of Kurucz model atmospheres. The chromospheric activitieswere determined by measuring the amount of emission in the cores of thestrong Caii HK lines. Results: Our comparison of the metallicity sampleto other [Fe/H] determinations was found to agree at the ±0.05dex level for spectroscopic values and at the ±0.1 dex level forphotometric values. The distribution of chromospheric activities isdescribed by a bimodal distribution, agreeing with the conclusions fromother works. Also an analysis of Maunder minimum status was attempted,and it was found that 6 ± 4 stars in the sample could be in aMaunder minimum phase of their evolution and hence the Sun should onlyspend a few per cent of its main sequence lifetime in Maunder minimum.Based on observations made with the ESO telescopes at the La SillaParanal observatory under programme ID's 076.C-0578(B) and077.C-0192(A). Table 4 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/485/571
| K dwarfs and the chemical evolution of the solar cylinder
K dwarfs have lifetimes older than the present age of the Galactic disc,and are thus ideal stars for investigating the chemical evolution of thedisc. We have developed several photometric metallicity indicators for Kdwarfs, based on a sample of accurate spectroscopic metallicities for 34disc and halo G and K dwarfs. The photometric metallicities lead us todevelop a metallicity index for K dwarfs based only on their position inthe colour-absolute-magnitude diagram. Metallicities have beendetermined for 431 single K dwarfs drawn from the Hipparcos catalogue,selecting the stars by absolute magnitude and removing multiple systems.The sample is essentially a complete reckoning of the metal content innearby K dwarfs. We use stellar isochrones to mark the stars by mass,and select a subset of 220 of the stars, which is complete within anarrow mass interval. We fit the data with a model of the chemicalevolution of the solar cylinder. We find that only a modest cosmicscatter is required to fit our age-metallicity relation. The modelassumes two main infall episodes for the formation of the halo-thickdisc and thin disc, respectively. The new data confirm that the solarneighbourhood formed on a long time-scale of the order of 7 Gyr.
| Long-term photometry of variables at ESO. I - The first data catalogue (1982-1986)
This paper presents the catalog of photometric data in the Stromgrensystem obtained during the first four years (October 1982 - September1986) of the Long-Term Photometry of Variables (LTPV) program at ESO.The data are available in computer-readable form.
| X-ray-selected M dwarfs and the diffuse X-ray background
A systematic optical identification program was carried out forserendipitous X-ray sources discovered by the Einstein Observatory. Atotal of 104 were detected above the threshold flux of 3 x 10 to the-13th ergs/sq cm per sec in the 114 sq deg survey region; 11 of these104 sources are dM stars. Including X-ray-selected M dwarfs from the LMCsurvey, the Medium Sensitivity Survey, and the Einstein Deep Survey, adwarf M star composite X-ray spectrum was constructed; the best-fitthermal model yields kT = 1.3 + or - 0.3 keV, consistent with typicalactive star spectra. An M dwarf X-ray luminosity function was alsoconstructed and it is concluded that the integrated contribution to thediffuse soft (0.28-1.0 keV) X-ray background from such stars is lessthan 10 percent, a factor of more than about 2 lower than previousestimates. This result is consistent with a limit on the dwarf M starcontribution derived from a spectral decomposition of the observed X-raybackground spectrum.
| An unbiased survey of field star X-ray emission
To determine the X-ray luminosity function of normal stars, 1700 starsbrighter than 10th magnitude were surveyed by the imaging proportionalcounter aboard the Einstein Observatory. Seventy star positions werefound to contain excess X-ray counting rates. The number of stars persquare degree in a number of magnitude intervals was calculated as afunction of spectral type and luminosity class, and the total number ofstars for each spectral type brighter than magnitude 9.5 derived in thismanner was compared with the 1700-star sample. The agreement is good, asis that between the surface density of soft X-ray sources and the numberof stellar emitters predicted from the field star survey. It isconcluded that stars probably do not contribute significantly to thediffuse soft X-ray background. The findings are consistent with thenotion that stellar age and/or rotation velocity are importantdeterminants of stellar X-ray emission level.
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
