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Post Info TOPIC: Red Dwarfs


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RE: Red Dwarfs

Three of the nearest red dwarf stars with planets all have less iron than the Sun, say astronomers in Texas. The discovery is a surprise, because heavy elements like iron make up the bulk of most planets in the solar system.
Most known extrasolar planets orbit stars with spectral types of F, G, or K, stars that are about as hot and luminous as the Sun. Not surprisingly, these planet-bearing Sunlike stars tend to have high abundances of heavy, planet-forming elements--abundances that match or surpass the Sun's.
But the few red dwarfs known to have planets violate this rule, say Jacob Bean, G. Fritz Benedict, and Michael Endl at the University of Texas at Austin. Red dwarfs are smaller, cooler, and fainter than the Sun. They account for three fourths of all stars in the Galaxy, including the Sun's nearest neighbour, Proxima Centauri. Yet red dwarfs glow so feebly that not a single one is visible to the unaided eye.

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Gliese 581



Posts: 131433

Title: Identification of New M Dwarfs in the Solar Neighbourhood
Authors: Basmah Riaz, John E. Gizis, James Harvin

Researchers present the results from a spectroscopic study of 1080 nearby active M dwarfs, selected by correlating the 2MASS and ROSAT catalogues. They have derived the spectral types and estimated distances for all of their stars. The spectral types range between K5 and M6. Nearly half of their stars lie within 50 pc. The researchers have measured the equivalent width of the H alpha emission line. Their targets show an increase in chromospheric activity from early to mid-spectral types, with a peak in activity around M5. Using the count rate and hardness ratios obtained from the ROSAT catalogue, they have derived the X-ray luminosities.
Their stars display a "saturation-type" relation between the chromospheric and coronal activity. The relation is such that log (Lx/Lbol) remains "saturated" at a value of about -3 for varying H alpha equivalent width. They have found 568 matches in the USNO-B catalogue, and have derived the tangential velocities for these stars. There is a slight trend of decreasing chromospheric activity with age, such that the stars with higher Vtan have lower H alpha equivalent widths. The coronal emission, however, remains saturated at a value of log (Lx/Lbol) -3 for varying tangential velocities, suggesting that the coronal activity remains saturated with age.
They do not find any break in the saturation-type relation at the spectral type where stars become fully convective (~M3.5). Most of the stars in their sample show more coronal emission than the dMe stars in the Hyades and Praesepe clusters, and have Vtan< 40km/s, suggesting a young population.

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