Issue |
A&A
Volume 520, September-October 2010
|
|
---|---|---|
Article Number | A91 | |
Number of page(s) | 5 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/201014419 | |
Published online | 08 October 2010 |
The magnetically-active, low-mass, triple
system WDS 19312+3607
(Research Note)
J. A. Caballero1,2 - D. Montes2 - A. Klutsch2 - J. Genebriera3 - F. X. Miret4 - T. Tobal4 - J. Cairol4 - S. Pedraz5,2
1 - Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir
km 4, 28850 Torrejón de Ardoz, Madrid, Spain
2 - Departamento de Astrofísica y Ciencias de la Atmósfera, Facultad de
Física, Universidad Complutense de Madrid, 28040 Madrid, Spain
3 - Observatorio de Tacande, La Palma, Spain
4 - Observatori Astronòmic del Garraf, Barcelona, Spain
5 - Centro Astronómico Hispano Alemán de Calar Alto (CSIC-MPG), c/
Jesús
Durbán Remón 2-2, 04004 Almería, Spain
Received 12 March 2010 / Accepted 7 June 2010
Abstract
Aims. We investigated in detail the system
WDS 19312+3607, whose primary is an active M4.5Ve star
previously inferred to be young (
-500 Ma) based on its
high X-ray luminosity.
Methods. We collected intermediate- and
low-resolution optical spectra taken with 2 m-class
telescopes, photometric data from the B to
8 m
bands, and data for eleven astrometric epochs with a time baseline of
over 56 years for the two components in the system, G 125-15
and G 125-14.
Results. We derived the M4.5V spectral types of both
stars, confirmed their common proper motion, estimated their
heliocentric distance and projected physical separation, determined
their Galactocentric space velocities, and deduced a most-probable age
of older than 600 Ma. We discovered that the primary,
G 125-15, is an inflated, double-lined, spectroscopic binary
with a short period of photometric variability of
1.6 d, which we associated with orbital synchronisation. The
observed X-ray and H
emissions, photometric variability, and abnormal radius and effective
temperature of G 125-15 AB are indicative of strong
magnetic activity, possibly because of the rapid rotation. In addition,
the estimated projected physical separation between
G 125-15 AB and G 125-14 of about
1200 AU ensures that WDS 19312+3607 is one of the
widest systems with intermediate M-type primaries.
Conclusions. G 125-15 AB is a
nearby (
26 pc), bright (
9.6 mag), active spectroscopic binary with a single
proper-motion companion of the same spectral type at a wide separation.
They are thus ideal targets for specific follow-ups to investigate wide
and close multiplicity or stellar expansion and surface cooling because
of the lower convective efficiency.
Key words: stars: activity - binaries: visual - binaries: spectroscopic - stars: individual: G 125-14 - stars: individual: G 125-15 - stars: low mass
1 Introduction
The binary WDS 19312+3607 (Washington Double Star identifier: GIC 158) is formed by the two nearby high proper-motion stars G 125-15 and G 125-14 (Giclas et al. 1971; Worley & Douglass 1997; Caballero et al. 2010). The primary, G 125-15, is an active M4.5Ve star with near-solar metal abundance (Reid et al. 2004). The secondary, G 125-14, is about 1 mag fainter in the visible and has never been investigated spectroscopically.
Interestingly, the system WDS 19312+3607 was
hypothesised to be a few
hundred million years old. Fuhrmeister & Schmitt (2003)
associated a ROSAT soft X-ray
source with G 125-15. Daemgen et al. (2007) and Allen
& Reid (2008)
inferred from its
location in a
versus V-J diagram that
G 125-15 has X-ray activity levels that exceed those of
Pleiades stars
of a similar spectral type and conservatively assumed an age of
300-500 Ma,
although the M dwarfs in their sample may be younger. Youth, closeness,
and late spectral type are optimal properties when
searching for faint companions to stars, ensuring that
G 125-15 became the
target of adaptive optics and IRAC/Spitzer
searches by Daemgen et al.
(2007) and
Allen & Reid (2008),
respectively. They provided restrictive upper limits to the magnitudes
and masses of
hypothetical brown dwarf and planetary companions at close
separations
(up to a few arcseconds). The secondary star, G 125-14, fell
out of the field of view of
Altair+NIRI/Gemini North in Daemgen et al. (2007), but is
among the brightest
sources in the IRAC/Spitzer images in Allen
& Reid (2008).
Both groups unintentionally overlooked the existence of the stellar
companion. They did not take into account the photometric variability
of the primary
either, which might be related to activity (and in turn to youth).
During the Hungarian Automated Telescope Network (HATnet) variability
survey in a field chosen to overlap with the Kepler
mission,
Hartman et al. (2004)
found G 125-15 to be a periodic variable with a pulsating
variable-like light curve. They measured a period
d
and an amplitude
mag.
The secondary star, G 125-14, was not analysed.
From the approximate angular separation of 47 arcsec
between G 125-15 and
G 125-14 and preliminary estimates of the heliocentric
distance to the
primary based on spectroscopic parallax (
15 pc - Reid et al. 2004;
Allen & Reid 2008),
we derived a rough projected physical separation
AU.
This wide separation and the late spectral type of the primary would
make the
system one of the widest low-mass binaries in the field (Caballero 2007,
2009;
Artigau et al. 2007;
Radigan et al. 2009).
If the age estimation by Daemgen et al. (2007) and Allen
& Reid (2008)
were correct, the WDS 19312+3607 system would be the first young
wide
low-mass binary in the solar neighbourhood. Thus, we aimed to
characterise this system in detail with new observations and
data compilation from the literature.
2 Observations and analysis
Table 1: Multi-epoch astrometric measurements of WDS 19312+3607.
We first used 11 astrometric epochs to measure the mean
angular
separation, position angle, and common proper motion of
G 125-15 and
G 125-14, as listed in Table 1.
We collected coordinates tabulated by the SDSS DR7, 2MASS, and
CMC14 catalogues,
and carried out standard astrometric analyses on public images (POSS,
IRAC) and
optical images obatined by us with CAFOS/2.2 m Calar Alto and 0.4 m
Tacande
. All the measurements are
consistent within 1
with a mean
angular separation
arcsec
and a position
angle
deg.
For comparison, during the 56.271 years of our time baseline,
the two stars
travelled together by about 10.3 arcsec. Using the methodology
presented in Caballero (2010),
we determined the proper
motion of the primary at
mas a-1,
which supersedes previous
determinations with larger uncertainties (Luyten 1979; Salim
& Gould 2003;
Hanson et al. 2004;
Lépine & Shara 2005;
Ivanov 2008).
Table 2: Basic data of G 125-15 AB and G 125-14.
We next compiled BVRI, ugriz,
,
and [3.6] [4.5][5.8][8.0]
photometric data of G 125-15 and G 125-14, which are
listed in
Table 2
with their associated uncertainties.
CAFOS images in the BVRI bands were calibrated
using stars in common with a
number of overlapping optical catalogues (Høg et al. 2000; Weis 1996;
Hartman et al. 2004).
We retrieved ugriz and
magnitudes and coordinates from the SDSS
and 2MASS catalogues, respectively (the SDSS iz
magnitudes of G 125-15 were
affected by saturation).
The magnitudes of G 125-15 in the four IRAC/Spitzer
channels were taken
from Allen & Reid (2008),
while those of G 125-14 were measured by us on
the public IRAC post-calibrated images.
On 2008 May 05, we used CAFOS with the grism
Blue-400 to acquire
low-resolution optical spectra (
at H
6562.8 Å)
of both G 125-15 and G 125-14. The two stars were
imaged simultaneously in the long slit (i.e., we did not
observe in parallactic angle). We also observed the late-type dwarf
FL Vir AB (M5.5Ve; Joy 1947) and
the spectrophotometric standard star HZ 44.
We carried out the bias correction, flat-fielding, spectra extraction,
wavelength calibration, and instrumental response correction following
standard
procedures within the astronomical data reduction package REDucmE
(Cardiel 1999)
. Our useful wavelength
coverage was from 4000 Å to 10 000 Å. The
final CAFOS spectra of G 125-15, G 125-14, and
FL Vir AB are shown in
Fig. 1.
From our data and classification based on pseudo-continuum indices
(e.g.,
Martín et al. 1999),
we agreed with the spectral type determination of the primary of M4.5Ve
by Reid et al. (2004).
The spectral types of G 125-15 and G 125-14 are
identical within an
uncertainty of 0.5 dex (Table 2).
![]() |
Figure 1: CAFOS/2.2 m Calar Alto spectra of FL Vir AB (in blue), G 125-15, and G 125-14 (in red), from top to bottom. They are normalised at 7500 Å and conveniently shifted in the vertical direction. |
Open with DEXTER |
Two new spectra were taken on 2009 Sep. 09 with the
Intermediate Dispersion
Spectrograph (IDS)
at the 2.5 m Isaac Newton Telescope (INT) on the Observatorio
del Roque de los Muchachos, La Palma, Spain.
In this case, we used the H1800V grating and the 0.95 arcsec
slit, which
provided a spectral resolution power of
,
and observed in
parallactic angle. With the same configuration, we also obtained
spectra of the comparison stars
GJ 687 (M3.5V) and GJ 1227
(M4.5V) and a number of
radial-velocity standards. The reduction and analysis of the data were
carried out using common tasks
within the IRAF envirnonment.
A part of the spectra of G 125-15, G 125-14, and
GJ 1227 around the
H
region is shown in Fig. 2.
The H
line in the intermediate-resolution spectrum of G 125-15 was
in
apparent, symmetric emission.
We measured a pseudo-equivalent width of pEW(H
) =
Å.
The line width at 10 % height was 3.0 Å,
significantly larger than those
of arc lines or H
emission lines in some active
late-type stars observed during the run with the same instrumental
configuration (of about 1.5 Å; Klutsch et al.,
in prep.). The absorption lines of G 125-15 appeared
remarkably to be double, which
implies that it is in turn a spectroscopic binary (SB2).
The apparent broadening of the H
line is more likely associated with the
partial overlapping of two non-broadened emission lines, one redshifted
and
other blueshifted, than to a process of accretion from a circumstellar
disc,
such as those found in classical T Tauri stars.
The consequences of the spectroscopic binarity of G 125-15
(hereafter
G 125-15 AB) are discussed in Sect. 3.
Besides this, we imposed a restrictive upper limit to the
pseudo-equivalent
width of the Li I
6707.8 Å
line.
This is not surprising, since M dwarfs destroy their lithium content in
20-150 Ma.
Similar upper limits were established for the H
and Li I lines in
G 125-14 (the H
line of the secondary is filled or in very faint
absorption).
The results are summarised in Table 2.
![]() |
Figure 2:
A 40 Å-wide region around the H |
Open with DEXTER |
Finally, we determined the radial heliocentric velocity of the three
components in WDS 19312+3607.
First, we analysed the cross-correlation functions of the IDS/INT
spectra
of G 125-15 AB, G 125-14, the comparison
stars, and radial-velocity
standard stars with the latest spectral types (about K7V) observed
during
our run. We found that the cross-correlation function of
G 125-15 AB compared to any
other single star observed with IDS had always two peaks, which is
consistent
with the primary having a double-lined spectrum and, hence, its
spectroscopic
binarity. We measured a radial velocity for G 125-14 with a
reasonable
precision of 2 km s-1
(Table 2),
the binarity of
G 125-15 AB and the proximity between the two peaks
in their cross-correlation
functions allowed us to determine the radial velocities of
G 125-15 A and B
with a precision about three times lower.
The difference in radial velocity between A and B was
40 km s-1 and
the mean (i.e., the radial velocity of the barycentre of A
and B) was
consistent with the radial velocity of G 125-14 within the
uncertainties.
In practice, we were unable to differentiate between the components
G 125-15 A and B because of the resemblance
between the depths of the double
lines in the spectrum and the heights of the two peaks in the
cross-correlation
functions. As a result, we assumed that A and B have the same
basic parameters (e.g., mass,
radius, effective temperature, magnitude, H
emission).
3 Discussion
3.1 Heliocentric distance
Allen & Reid (2008)
derived pc
to G 125-15 AB
assuming singleness and normal radius and effective temperature (Reid
et al.
2004, had
derived
pc
to G 125-15 AB, but also
pc
to G 125-14 based on an incorrect V
magnitude). This implies a projected physical separation between
G 125-15 AB and
G 125-14 of
AU.
There are numerous absolute magnitude-spectral type relations
useful for
determining heliocentric distances of intermediate- and late-M field
dwarfs
without parallax measurement (e.g., Henry et al. 1994; Hawley
et al. 2002;
Cruz et al. 2003;
Phan-Bao & Bessell 2006;
Caballero et al. 2008).
In this work, we used the MJ-Sp.
type relation of Scholz et al. (2005),
which is given in spectral type intervals of 0.5 dex.
The derived absolute magnitude of G 125-14 was mag.
For the computation, we could only use the secondary G 125-14
because the
absolute magnitude of the primary G 125-15 AB is
affected by spectroscopic
binary and activity (see Sect. 3.2).
Using the value of MJ,
the 2MASS J-band magnitude of G 125-14 in
Table 2,
and the Pogson law,
,
and
accounting for the scatter in the MJ-Sp.
type relation, we
estimated a heliocentric distance of
pc.
At this distance, the angular separation between
G 125-15 AB and G 125-14
translates into a projected physical separation of
AU,
which ensures that WDS 19312+3607 is one of the
brightest, closest, low-mass systems with very low binding energies.
3.2 Close binarity and magnetic activity
From Table 2,
the primary is 1.0-1.5 mag brighter than the
secondary depending on the passband, while they have the same spectral
type
within a 0.5 dex uncertainty. The equal-mass binarity of the
primary accounts for only about 0.75 mag (
).
Since the stars are located at the same short heliocentric distance,
the
primary displays a wavelength-dependent overbrightness of
0.3-0.8 mag.
In addition, G 125-15 AB is redder
than G 125-14.
For example, the difference in r-J
colours, which depends strongly on the
effective temperature, is
mag.
This deviation is marginally consistent within the 0.5 dex
uncertainty in
spectral type determination, but not with the observed overbrightness
of
0.3-0.8 mag.
We estimated the ratios of effective temperatures and radii
needed to
explain the observed magnitude and colour differences between
G 125-15 AB and
G 125-14. The ratio of the sum of observed fluxes at the B
to [8.0] bands is
(using
and
the corresponding zero-point conversion factors
),
where
``(1)'' and ``(2)'' indicate G 125-15 AB and
G 125-14, respectively. This quotient is a reasonable
approximation to the ratio of total luminosities,
L(1)/L(2),
from where one derives
3.2 after assuming that
the two components in G 125-15 AB have the same mass
and effective temperature.
A cooler effective temperature, indicated by a redder r-J
colour, must be
counterbalanced by a larger radius.
We estimated that the two components in G 125-15 AB
are
%
cooler and
%
larger than normal M4.5
dwarfs (including G 125-14), which have
-3300 K
and
-0.26
.
Effective temperature variations larger than 5% would lead to a
different
spectral type classification of G 125-15 AB and
G 125-14.
Radii and effective temperatures in M dwarfs are affected by
activity levels
(Stauffer & Hartmann 1986;
Mullan & MacDonald 2001;
Torres & Ribas 2002;
López-Morales 2007;
Reiners et al. 2007;
Morales et al. 2008).
According to Chabrier et al. (2007), the
lower heat fluxes and, thus, larger
radii and cooler effective temperatures of active low-mass stars and
brown
dwarfs compared to regular (inactive) stars are caused by the reduced
convective efficiency, produced by the rapid rotation and high field
strengths,
and/or to magnetic spot coverage of the radiating surface.
Previously, the activity scenario of G 125-15 AB was
only consistent with the
high relative X-ray flux (Daemgen et al. 2007; Allen
& Reid 2008).
We now find that its is consistent with its H
emission (Reid et al.
2004; this
work), stellar expansion (by about 30%; this work), and
photometric variability (Hartman et al. 2004). This
variability is more easily explained by an asymmetrical distribution
of cool spots concentrated in certain hemispheres of two close,
magnetically-active, orbital-locked, M4.5Ve stars rather than by
pulsations in a
low-mass dwarf. The period observed by Hartman et al. (2004) would be
the rotational period of
the system at
d.
This value is quite short for field M dwarfs and indicative of rapid
rotation,
as expected by the Chabrier et al. (2007)
scenario.
Table 3: Properties of the WDS 19312+3607 system.
3.3 Space motion, age, mass, and semimajor axis
We assumed that the strong magnetic activity in
G 125-15 AB is not due to
youth, as previously understood, but to rapid rotation in a close
orbital-locked
system. First, if it were young, G 125-14 should also display
signposts of youth.
Second, we derived the Galactocentric space velocities UVW
of the
WDS 19312+3607 system (Table 3) as in Montes
et al. (2001).
In the U-V and U-W
diagrams, WDS 19312+3607 lies outside the
region that includes young moving groups with ages from Ma
(e.g., TW Hydra,
Pictoris,
AB Doradus) to
Ma
(e.g., Castor, Hyades). However, the UVW velocities
of WDS 19312+3607 are very different from those of
old-disc stars (Leggett 1992).
The most probable age of G 125-15 AB and
G 125-14 from kinematics criteria
is thus
Ga.
We estimated the semimajor axis of the close binary
G 125-15 AB assuming that
the orbital period coincides with the photometric one.
Before applying Kepler's third law, we had to estimate the masses of
each
component in the system from their absolute magnitudes and theoretical
models.
We determined the mass of G 125-14, the only normal single
dwarf in the system,
at about 0.18
using its MJ
magnitude (Table 3)
and
NextGen theoretical isochrones (Baraffe et al. 1998), which
are only weakly
sensitive to age if 0.3 Ga
10 Ga.
Based on the resemblance of spectral types, we cautiously assigned
similar
masses to the components in G 125-15 AB.
Using these masses and the rotational-orbital period of the system, we
estimated
that the two stars are separated by only
AU
(
or about 10-20 stellar radii).
The estimated semimajor axis a is very
short for M dwarfs and comparable to
that of the well-known CM Dra system, which is
formed by two
population II M4.5 dwarfs (Lacy 1977; Vilhu
et al. 1989;
Chabrier & Baraffe
1995;
Metcalfe et al. 1996;
Viti et al. 1997;
Doyle et al. 2000;
Morales et al.
2009). The
two flaring stars in CM Dra are separated by AU
and have
a rapid tidally-synchronised rotation period of 1.27 d,
slightly shorter than
the photometric period of G 125-15 AB.
Because of its high inclination angle of i =
89.82 deg, CM Dra is an
eclipsing binary.
In analogy, the probability of eclipsing in
G 125-15 AB must be relatively
high, at about 10-20% (as inferred from the ratio
,
where
is the
radius of the two components).
If the individual masses in G 125-15 AB were lower
than expected for its
spectral type due to activity (as seen in CM Dra - Lacy 1977), the
semimajor
axis would be shorter than 0.019 AU and the probability of
eclipsing would
increase.
Radii, masses, and effective temperatures of the two
components in CM Dra, as
well as in other eclipsing binaries, are widely used to compare
observations to theoretical models. In contrast to CM Dra,
which has a white-dwarf proper-motion companion at
26 arcsec, G 125-15 AB has a wide
proper-motion companion, G 125-14, that is
a dwarf of the same spectral type within an uncertainty of
0.5 dex. The three stars can be used to
study properly the relation between both
stellar radius and effective temperature and activity at the bottom of
the main
sequence. As discussed in Sect. 3.2,
G 125-15 AB and G 125-14 also
exhibit a temperature reversal with a relative amplitude of 5%.
These temperature reversals have been also detected in other
cornerstone active
M-type eclipsing binaries, such as the young brown-dwarf pair
2MASS J05352184-0546085 (Stassun et al. 2006, 2007).
4 Summary
Daemgen et al. (2007)
and Allen & Reid (2008)
proposed that G 125-15 is
a single, active, M4.5Ve-type star in the solar neighbourhood younger
than the
Hyades (
600 Ma) based mainly on strong X-ray activity detected by
Fuhrmeister & Schmitt (2003). The
dwarf is instead part of the wide binary system candidate
WDS 19312+3607,
which was tabulated earlier by Giclas et al. (1971). Its
proper-motion companion candidate is G 125-14, a poorly-known
late-type dwarf more than 1.0 mag fainter located about
46 arcsec to the
north. To test the youth and wide-binarity hypotheses, we carried out
spectroscopic, photometric, and astrometric analyses of the system
using a
collection of multiwavelength public and private data.
We found that the primary is a spectroscopic binary with Hin broad
emission and concluded that G 125-15 AB and
G 125-14 form a
0.6-5 Ga-old hierarchical triple system at about
26 pc from the Sun. The three components each have estimated
masses of 0.18
.
While G 125-15 AB and G 125-14 are separated
by
arcsec,
which translates into a wide projected physical
separation of 1200
+600-300 AU,
G 125-15 A and B are separated
by only about 0.02 AU.
This close separation is responsible for the synchronisation of the
pair and,
thus, a short rotational period identical to the observed photometric
period of
d.
Rapid rotation accounts for the higher magnetic activity of the pair,
which is illustrated by its strong X-ray activity, H
emission,
photometric variability (possibly associated with the presence of cool
spots),
and, in particular, larger radii of the two components, with respect to
normal dwarfs of the same spectral type.
The brightness and proximity of WDS 19312+3607 will facilitate astrometric, photometric, and spectroscopic follow-up studies, particularly those designed to determine accurate trigonometric parallax, age, and radial and rotational velocities of the system, and investigate the relation between radius, effective temperature, and magnetic activity.
AcknowledgementsWe thank the anonymous referee for helpful comments and P. G. Pérez-González for software help. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía. Based on observations made with the Isaac Newton Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This research made use of the SIMBAD, operated at Centre de Données astronomiques de Strasbourg, France, and NASA's Astrophysics Data System. Financial support was provided by the Universidad Complutense de Madrid, the Comunidad Autónoma de Madrid, and the Spanish Ministerio deCiencia e Innovación under grants AyA2008-00695, AyA2008-06423-C03-03, and SP2009/ESP-1496.
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Footnotes
- ... Alto
- http://www.caha.es/alises/cafos/cafos.html
- ...
Tacande
- http://www.astropalma.com/astropalma_eng.htm
- ...1999)
- http://www.ucm.es/info/Astrof/software/reduceme/reduceme.html
- ... (IDS)
- http://www.ing.iac.es/Astronomy/instruments/ids/
- ... factors
- http://nsted.ipac.caltech.edu/NStED/docs/parhelp/Photometry.html
All Tables
Table 1: Multi-epoch astrometric measurements of WDS 19312+3607.
Table 2: Basic data of G 125-15 AB and G 125-14.
Table 3: Properties of the WDS 19312+3607 system.
All Figures
![]() |
Figure 1: CAFOS/2.2 m Calar Alto spectra of FL Vir AB (in blue), G 125-15, and G 125-14 (in red), from top to bottom. They are normalised at 7500 Å and conveniently shifted in the vertical direction. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
A 40 Å-wide region around the H |
Open with DEXTER | |
In the text |
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