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The discovery of a new binary star system candidate in Virgo consisting of two M type dwarfs is described. A variety of astronomical characteristics are used to describe the two components of the system including absolute magnitude, distance and spectral type
A new binary system in Virgo?
Martin Nicholson, Daventry, England
The discovery of a new binary star system candidate in Virgo consisting of two M type dwarfs is described. A variety of astronomical characteristics are used to describe the two components of the system including absolute magnitude, distance and spectral type.
Target Identification via proper motion data
Proper-motion catalog SDSS + USNO-B (Gould+, 2004)
Table 1a - Published proper motion data on the pair
Proper motion in RA mas/yr
Proper motion in DE mas/yr
Total Pm mas/yr
Table 1b - Summary of the SDSS data
| umag|| gmag|| rmag|| imag|| zmag|
11 33 37.02 +00 35 20.6
11 33 37.52 +00 35 14.8
This corresponds to a separation of 9.535 arc sec at a position angle of 128.256 degrees and at date 1999.2178 .
Manipulation of the data in the SDSS catalogue along the lines suggested by Martín (2006) allows other key facts about the two stars to be determined. This helps to resolve the problem of distinguishing between gravitationally bound stars and simple line-of-sight arrangements.
This is a six stage process:
- Use SDSS u, g, r, i, z magnitudes to calculate B, V, R and I magnitudes.
- Calculate B-V and V-I colour indices.
- Use SDSS g, r and i magnitudes to calculate absolute magnitude in g band (Mg).
- Use Mg to calculate absolute visual magnitude (Mv).
- Use V and Mv to calculate distance in parsecs.
- Use Mv to calculate spectral type and mass.
Stages 1 and 2
The transformations suggested by Lupton (2005) allow the B V R and I magnitudes to be calculated from the quoted SDSS magnitudes.
B = u - 0.8116*(u - g) + 0.1313
B = g + 0.3130*(g - r) + 0.2271
V = g - 0.2906*(u - g) + 0.0885
V = g - 0.5784*(g - r) - 0.0038
R = r - 0.1837*(g - r) - 0.0971
R = r - 0.2936*(r - i) - 0.1439
I = r - 1.2444*(r - i) - 0.3820
I = i - 0.3780*(i - z) - 0.3974
Table 2 - Derived magnitudes from the SDSS data
Stages 3 and 4
A paper by Bilir et al (2005) links SDSS magnitudes to the absolute magnitude in the g band (Mg) and another by Karaali et al. (2005) links the absolute magnitude in the g band to the absolute visual magnitude (Mv).
Mg = a(g - r) + b(r - i) + c (where a = 5.791, b = 1.242 and c = 1.412)
Mv = 0.9972 (Mg) - 0.046
Hence the primary star has Mg = 10.634 and Mv = 10.558
The secondary star has Mg = 10.866 and Mv = 10.790
The absolute magnitude (Mv), V band magnitude (V) and distance in parsecs (d) are linked by the standard expression:
V - Mv = 5 log d - 5
Table 3 - Derived distances of the two components
Distance in parsecs
In Henry et al (1994) the spectral type (ST) of M dwarfs is connected to the absolute visual magnitude by the equation:
Mv = 0.101(ST)2 + 0.596(ST) + 8.96
Substituting into this equation gives the primary star a spectral type of M2.0V and the secondary star a spectral type of M2.2V with an uncertainty of +/- 0.5 spectral sub-classes.
Delfosse et al (2000) derived an equation linking absolute magnitude and mass.
Log (Star mass/Solar mass) = 0.001(0.3+1.87Mv+7.614Mv2-1.698Mv3+0.060958Mv4)
The primary star has a mass of 0.424 solar masses and the secondary 0.398 solar masses.
The available evidence would appear to support the hypothesis of a physical link between the two stars. The two stars are of almost identical spectral types, at almost identical distances (179 parsecs) and are very close together ( 9.535 arc second) in the sky.
It is hoped that this new double will be included in the WDS.
Funding for the SDSS-II has been provided by the Alfred P. Sloan Foundation, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England.
The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions: the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, Cambridge University, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPA), the Max-Planck-Institute for Astrophysics (MPIA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.
The author also made use of the following on-line resources :-
The Vizier catalogue service http://vizier.u-strasbg.fr/
Proper-motion catalog SDSS + USNO-B (Gould+, 2004)
Lupton, R., 2005, Transformations between SDSS magnitudes and UBVRcIc, http://www.sdss.org/dr4/algorithms/sdssUBVRITransform.html#Lupton2005
Bilir, S.; Karaali, S.; Tunçel, S., 2005, Absolute magnitudes for late-type dwarf stars for Sloan photometry, Astronomische Nachrichten, Vol.326, Issue 5, 321-331.
Delfosse, X.; Forveille, T.; Ségransan, D.; Beuzit, J.-L.; Udry, S.; Perrier, C.; Mayor, M., 2000, Accurate masses of very low mass stars. IV. Improved mass-luminosity relations, Astronomy and Astrophysics, v.364, 217-224.
Henry, Todd J.; Kirkpatrick, J. Davy; Simons, Douglas A., 1994, The solar neighborhood. 1. Standard Spectral Types (K5-M8) for northern dwarfs within eight parsecs , The Astronomical Journal, vol. 108, no. 4, 1437-1444.
Karaali, S.; Bilir, S.; Tunçel, S., 2005, New Colour Transformations for the Sloan Photometry, and Revised Metallicity Calibration and Equations for Photometric Parallax Estimation, Publications of the Astronomical Society of Australia, Volume 22, Issue 1, 24-28.
Martín, Edgardo Rubén Masa, 2006, SDSS J001708.1-102649.5 & SDSS J001707.99-102647.3: Serendipitous Discovery of a New Binary System Candidate, Journal of Double Star Observations, vol. 3, no. 1, 34-48.
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