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Flares May Threaten Planet Habitability Near Red Dwarfs

A team of scientists has combed 10 years of ultraviolet observations by NASA's Galaxy Evolution Explorer (GALEX) spacecraft looking for rapid increases in the brightness of stars due to flares. Flares emit radiation across a wide swath of wavelengths, with a significant fraction of their total energy released in the ultraviolet bands where GALEX observed. At the same time, the red dwarfs from which the flares arise are relatively dim in ultraviolet. This contrast, combined with the GALEX detectors' sensitivity to fast changes, allowed the team to measure events with less total energy than many previously detected flares. This is important because, although individually less energetic and therefore less hostile to life, smaller flares might be much more frequent and add up over time to create an inhospitable environment.
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Title: A Spectroscopic Catalogue of the Brightest (J<9) M Dwarfs in the Northern Sky
Authors: Sebastien Lepine, Eric J. Hilton, Andrew W. Mann, Matthew Wilde, Barbara Rojas-Ayala, Kelle L. Cruz, Eric Gaidos

We present a spectroscopic catalogue of the 1,556 brightest M dwarf candidates in the northern sky, as selected by proper motion and photometry. These bright sources comprise >99% of the known, northern M dwarfs with apparent magnitudes J95% of all such existing objects. Only 679 stars in our sample are listed in the Third Catalogue of Nearby Stars (CNS3); most others are relative unknowns and have spectroscopic data presented here for the first time. Observations confirm 1,403 of the candidates to be late-K and M dwarfs with spectral subtypes K7-M6, with subtypes assigned based on spectral index measurements of CaH and TiO molecular bands. We also calculate the Zeta parameter, which measures the ratio of TiO and CaH bandheads, and is correlated with metallicity in M dwarfs/subdwarfs, and for this we present a revised calibration based on corrected values of the CaH and TiO spectral indices. Fits of our spectra to the Phoenix atmospheric model grid are used to estimate effective temperatures. Existing geometric parallax measurements for 624 of the catalogue stars are used to recalibrate the subtype/absolute magnitude relationship in M dwarfs; we find that spectroscopic distances are marginally more accurate at earlier (K7-M2) subtypes, but that photometric distances should be preferred for later-type dwarfs (M3-M6). We identify active stars from H\alpha equivalent widths, GALEX UV magnitudes, and ROSAT X-ray fluxes from ROSAT. We combine proper motion data and photometric distances to evaluate the distribution in (U,V,W) velocity space of the entire catalogue. The overall velocity-space distribution correlates tightly with the velocity distribution of G dwarfs in the Solar Neighbourhood. However, active stars show a smaller dispersion in their space velocities, which is consistent with those stars being younger on average.

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Nearby red dwarfs
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Google earth file: Nearby red dwarfs.kmz (3kb, kmz)



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NASA's Hubble Finds that Puny Stars Pack a Big Punch

A survey of more than 200,000 stars in our Milky Way galaxy has unveiled the sometimes petulant behaviour of tiny red dwarf stars. These stars, which are smaller than the Sun, can unleash powerful eruptions called flares that may release the energy of more than 100 million atomic bombs. Red dwarfs are the most abundant stars in our universe and are presumably hosts to numerous planets. However, their erratic behaviour could make life unpleasant, if not impossible, for many alien worlds. The flares the stars unleash would blast any planets orbiting them with ultraviolet light, bursts of X-rays, and a gush of charged particles called a stellar wind.
Studying the light from 215,000 red dwarfs collected in observations by NASA's Hubble Space Telescope, astronomers found 100 stellar flares. The observations, taken over a seven-day period, constitute the largest continuous monitoring of red dwarf stars ever undertaken. The illustration shows a red dwarf star unleashing a powerful flare. A hypothetical planet is in the foreground.

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The Universe Does Think Small

The biggest galaxies in the universe are elliptical galaxies. The largest of these hold over one trillion stars according to astronomical census takers, compared to 400 billion in our Milky Way. However, new research shows that elliptical galaxies actually hold five to ten times as many stars as previously believed. This means that the total number of stars in the universe is likely three times bigger than realised.
The hidden stars are known as red dwarfs for their colour and small size. Because red dwarfs are small and dim compared to stars like the Sun, astronomers hadn't been able to detect them in galaxies beyond the Milky Way before now. As such, they didn't know how many stars in the universe were red dwarfs.

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Universe might hold three times more stars than previously thought

It's a cosmic embarrassment of riches - the universe appears to hold three times the number of stars many astronomers might have estimated only a year ago.
That's the implication a pair of scientists has drawn after measuring eight huge elliptical galaxies that they selected from two vast galaxy clusters located between 53 million to 321 million light-years from Earth.
With as many as 200 billion galaxies in the observable universe, each with hundreds of billions of stars, the result - if it holds up - implies an enormous number of additional burning gas balls out there, with intriguing implications for explanations of how stars and galaxies form and evolve, researchers say.

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Discovery Triples Number of Stars in Universe

Astronomers have discovered that small, dim stars known as red dwarfs are much more prolific than previously thought - so much so that the total number of stars in the universe is likely three times bigger than realised.
Because red dwarfs are relatively small and dim compared to stars like our Sun, astronomers hadnt been able to detect them in galaxies other than our own Milky Way and its nearest neighbours before now. As such, they did not know how much of the total stellar population of the universe is made up of red dwarfs.

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M dwarfs
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Roughly three quarters of the stars in the galaxy are red dwarfs, but planet searches have typically passed over these tiny faint stars because they were thought to be unfriendly to potential life forms.
But this prejudice has softened lately. Preliminary results from a dedicated research program have shown that planets around red dwarfs could be habitable if they can maintain a magnetic field for a few billion years.
Red dwarfs - also called M dwarfs - are between 7 and 60 percent as massive as our sun. Their lower mass means they don't burn as hot or as brightly, emitting less than 5 percent as much light as the sun. However, they have strong magnetic activity, which makes them relatively bright in X-rays and UV radiation and causes them to flare frequently.

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Red Dwarfs
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Title: The "Living with a Red Dwarf" Program
Authors: Edward F. Guinan, Scott G. Engle

Red Dwarfs (main-sequence / dwarf M or dM) stars are the most common stars in the Galaxy. These cool, faint, low mass stars comprise over 75% of all stars. Because of their low luminosities (~0.0008-0.06 of the Sun's luminosity), the circumstellar habitable zones (HZs) of dM stars are located within ~0.05-0.4 AU of the host star. Nevertheless, the prospect of life on a planet located within the HZ of a red dwarf is moderately high, based on the longevity of these stars (>50 Gyr), their constant luminosities and high space densities. Here we describe the aims and early results of the "Living with a Red Dwarf" Program - a study of dM stars that we have been carrying out over the last few years. The primary focus of our research on dM stars is the study of their magnetic dynamos and resulting star spots & coronal X-ray and chromospheric UV emissions as a function of age, rotation and spectral type. This program will provide datasets that can be used as inputs for the study of all aspects of dM stars, along with the planets already discovered hosted by them and the probable hundreds (thousands?) of planets expected to be uncovered in the near future by missions such as Kepler & Darwin/TPF. These datasets will be invaluable to those who model exo-planetary atmospheres, as well as exobiologists & astrobiologists who are studying the possibilities of life elsewhere in the universe. A significant element of our program is the determination of accurate stellar magnetic-driven X-ray-UV (X-UV) irradiances that are generated by the dM stars' vigorous magnetic dynamos. These X-UV irradiances (and flare frequencies) are strongly dependent on rotation, and thus age, and diminish as the stars lose angular momentum and spin-down over time via magnetic braking.

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M Dwarfs
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Title: Spitzer-IRAC Search for Companions to Nearby, Young M Dwarfs
Authors: Peter R. Allen, I. Neill Reid

We present the results of a survey of nearby, young M stars for wide low-mass companions with IRAC on the Spitzer Space Telescope. We observed 40 young M dwarfs within 20 pc of the Sun, selected through X-ray emission criteria. A total of 10 candidate companions were found with IRAC colours consistent with T dwarfs. Extensive ground-based NIR follow-up observations rejected all these candidates. Two additional candidates were discovered via common proper motion measurements, one of which was rejected as a background object and the other is a bona fide companion to GJ 2060, a member of the AB Doradus moving group.
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