From: Amara Graps (amara@amara.com)
Date: Thu Feb 13 2003 - 07:11:41 MST
http://xxx.lanl.gov/abs/astro-ph/0302206
Some things about this new star and its discovery that I think are
interesting:
It was found in a database of near-earth-asteroid-tracking (NEAT)
objects, so it's ideally suited for large proper motions. (the
tangential velocity was measured to be 52.9 +/- 11.7 km/sec.
HPMS is what looks? to be a normal metal (second generation?) red dwarf.
The 'underluminous' is interesting, but it seems in relation to a couple
of standard stars (dwarfs) so I'm not sure anyone can say very much yet
about that aspect.
the authors say:
"We identify the HPMS as a main-sequence dwarf with spectral type
M6.5 V based upon detailed examination of the CaH2 CaH3 and TiO5
spectral features \citep{ref10}. The relative strengths of the CaH
and TiO bands show no evidence that the star is significantly
metal-poor."
If you look on my HR diagram below, you can see that where the
star is evolving off the main sequence from the extreme lower right.
[Absolute magnitude of 10.6 and type M6.5 is the lower right
on this chart:
http://www.amara.com/past/HRDiagmed.gif
from the Web page: http://www.amara.com/past/irexcess.html
Note, the line is an empirical fit to data as described in
http://www.amara.com/ftpstuff/main-sequence.txt]
They also say in their paper:
"If our trigonometric parallax is accurate, then the HPMS is
underluminous relative to a normal M6.5 disk dwarf by 1.2+/- 0.7
magnitudes. This result might be expected for an extreme metal-poor
subdwarf, such as LHS 1826 \citep{ref12}, but, as noted above, there
is no indication that HPMS is metal-poor. If the star has the same
effective temperature as GJ 1111, then the low luminosity implies that
the HPMS has a radius only 60% that of GJ 1111, or ~0.68
R_{Jupiter}, violating electron degeneracy.
Given these results, we have estimated a spectrophotometric parallax
for the HPMS. \citet{ref11} have shown that spectral type is well
correlated with MJ for ultra-cool dwarfs. Using their linear relation,
we derive MJ= 10.59 +/- 0.25, corresponding to a distance of
3.6 +/- 0.4 parsecs. At this distance, which is almost identical to
that of GJ 1111, HPMS is the 17th nearest system (and 27th nearest
star) to the Sun."
Two useful tables from their paper (I stripped out most of the TeX notation)
HPMS Parameters
RA (J2000){a} 42.24369 deg
DEC (J2000){a}Ý 16.89200 deg
Epoch {a} 1989.84
Magnitudes (B,V, R, I, J, H, K( _{s} )){b}
(17.21 +/- 0.11, 15.40 +/- 0.08, 13.26 +/- .06, 10.66 +/- 0.03,
8.39 +/- 0.03, 7.88 +/- 0.04, 7.59 +/- 0.05 )
Spectral Type M6.5 V
Proper Motion 5.06 +/- 0.03 arcsec/yr
Position Angle 138.1 +/- 0.3 deg
Parallax pi 0.43 +/- 0.13 arcsec
Distance (1/( pi )) 2.3 (+1.0,-0.5) pc
Tangential Velocity 52.9 +/- 11.7 km/s
{a}{From 2nd Guide Star Catalogue}
{b}{B,V,R,I are from this work. J, H, K$_{s}$ are from the 2MASS
Catalogue.}
AND
Nearest Known Stars {a}
{Name} {D(pc)} {PM{b} {M(V){c}} {Sp. Type}
(arcsec/yr)}
Proxima Centauri 1.30 3.81 15.49 dM5 e
Alpha Centauri A 1.34 3.69 4.38 G2 V
Alpha Centauri B 1.34 3.69 5.71 K0 V
Barnards Star 1.83 10.31 13.23 M5 V
***{HPMS (this work)} 2.3 (+1.0,-0.5) 5.06+/-0.03 18.5+/-0.7 M6.5 V
Wolf 359 2.39 4.70 16.56 M6
Gl 411 2.52 4.81 10.48 M2 Ve
Luyten 726-8 A 2.63 3.37 15.47 dM5.5e
Luyten 726-8 B (UV Ceti) 2.63 3.37 15.60 dM5.5e
Sirius A 2.63 1.33 1.47 A1 V
Sirius B 2.63 1.33 11.34 DA2
{a}{All values in this table (with the exception of the HPMS) were
either taken directly or derived from the Preliminary Version of the
Third Catalogue
of Nearby Stars.{ref2}}
{b}{Proper motion.}
{c}{Absolute visual magnitude.}
-- Amara Graps, PhD Istituto di Fisica delle Spazio Interplanetario (IFSI) Consiglio Nazionale delle Ricerche (CNR), Roma, ITALIA Amara.Graps@ifsi.rm.cnr.it
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