A new L dwarf member of the moderately metal poor... HD 221356

A new L dwarf member of the moderately metal poor... HD 221356
Mon. Not. R. Astron. Soc. 427, 2457–2463 (2012)
doi:10.1111/j.1365-2966.2012.22113.x
A new L dwarf member of the moderately metal poor triple system
HD 221356
B. Gauza,1,2 V. J. S. Béjar,1,2 R. Rebolo,1,2,3 K. Peña Ramı́rez,1,2
M. R. Zapatero Osorio,4 A. Pérez-Garrido,5 N. Lodieu,1,2 D. J. Pinfield,6
R. G. McMahon,7,8 E. González-Solares,7 J. P. Emerson,9 S. Boudreault1,2
and M. Banerji7
1 Instituto
de Astrofı́sica de Canarias (IAC), E-38200 La Laguna, Tenerife, Spain
Astrofı́sica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain
3 Consejo Superior de Investigaciones Cientı́ficas, E-28006 Madrid, Spain
4 Centro de Astrobiologı́a (CSIC-INTA), E-28850 Torrejón de Ardoz, Madrid, Spain
5 Universidad Politécnica de Cartagena, Campus Muralla del Mar, Cartagena, Murcia E-30202, Spain
6 Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB
7 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA
8 Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA
9 Astronomy Unit, School of Physics & Astronomy, Queen Mary University of London, London E1 4NS
2 Department
ABSTRACT
We report on the discovery of a fourth component in the HD 221356 star system, previously
known to be formed by an F8V, a slightly metal poor primary ([Fe/H] = −0.26), and a distant
M8V+L3V pair. In our ongoing common proper motion search based on Visible and Infrared
Survey Telescope for Astronomy (VISTA) Hemisphere Survey (VHS) and Two Micron All
Sky Survey (2MASS) catalogues, we have detected a faint J = 13.76 ± 0.04 mag comoving
companion of the F8 star located at angular separation of 12.13 ± 0.18 arcsec (position
angle of 221.◦ 8 ± 1.◦ 7), corresponding to a projected distance of ∼317 au at 26 pc. Nearinfrared spectroscopy of the new companion, covering the 1.5–2.4 µm wavelength range with
a resolving power of R ∼ 600, indicates an L1 ± 1 spectral type. Using evolutionary models
the mass of the new companion is estimated at ∼0.08 M , which places the object close
to the stellar–substellar borderline. This multiple system provides an interesting example of
objects with masses slightly above and below the hydrogen-burning mass limit. The low-mass
companions of HD 221356 have slightly bluer colours than field dwarfs with similar spectral
type, which is likely a consequence of the subsolar metallicity of the system.
Key words: brown dwarfs – stars: individual: HD 221356 – stars: low-mass.
1 I N T RO D U C T I O N
Because of progressive cooling with age, brown dwarfs do not obey
a unique mass–luminosity relation (Burrows et al. 1997, 2001).
Therefore, the determination of a brown dwarf mass requires either
a good knowledge of its age or a direct dynamical measurement.
This, in turn, is possible for substellar companions of stars or for
those that are found in multiple systems. An additional advantage is
that the metallicity can be inferred from the primary star. For solartype stars the atmospheres are much better understood than for
very low mass stars and brown dwarfs, given the poor knowledge of
E-mail: [email protected]
C 2012 The Authors
C 2012 RAS
Monthly Notices of the Royal Astronomical Society opacities in cool atmospheres (Bonfils et al. 2005; Bean et al. 2006).
Coeval systems containing low-mass companions also provide very
useful constraints on evolutionary and atmospheric models (e.g.
Pinfield et al. 2006; Dupuy et al. 2010) as well as offering a rather
unique view on how the process of star formation works at the very
bottom of the main sequence (Burgasser et al. 2007). In particular,
brown dwarf companions with well-determined metallicities are
benchmark objects allowing for a better understanding of the effects
of metallicity on the physical properties and evolution of substellar
objects (Pinfield et al. 2012). Unfortunately, substellar companions
located at wide separations (>50 au) from their parent stars are
relatively rare, with an estimated frequency of less than a few per
cent (McCarthy & Zuckerman 2004; Kraus & Hillenbrand 2007;
Lafrenière et al. 2007).
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Accepted 2012 September 11. Received 2012 August 27; in original form 2012 June 22
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B. Gauza et al.
Table 1. Properties of HD 221356A (also known as
HR 8931, HIP 116106).
Parameter
Value
RA (J2000)
Dec. (J2000)
V (mag)
Spectral type
μα cos (δ)
μδ
Parallax
Distance
T eff
log (g)
Mass
[Fe/H]
Age
23h 31m 31.s 62
−04◦ 05 16. 78
6.50
F8V
178.7 ± 0.9 mas yr−1
−192.8 ± 0.9 mas yr−1
38.29 ± 0.54 mas
26.12 ± 0.37 pc
5976 ± 44 K
4.31 ± 0.06 cm s−2
0.94 ± 0.13 M
−0.26 ± 0.03
2.5–7.9 Gyr
References
–
–
1
3
2
2
2
2
1
1
1
1
1
(∼20 000 deg2 ) in the JK s broad-band filters with a sensitivity more
than 3 mag deeper than 2MASS. It uses the 4.1-m telescope VISTA
operating since 2009 at ESO’s Cerro Paranal Observatory in Chile
(Emerson, McPherson & Sutherland 2006). It is equipped with
a wide-field near-infrared camera (VISTA Infrared Camera, VIRCAM), comprising 16 ‘2k × 2k pixel’ detectors with a mean plate
scale of 0.339 arcsec. The HD 221356 system was observed with
VISTA on 2010 November 25 and 26. Average seeing conditions
were 1.4 and 0.9 arcsec, respectively.
The VHS near-infrared images are processed and calibrated automatically by a dedicated science pipeline implemented by the
Cambridge Astronomical Survey Unit (CASU). Standard reduction and processing steps include dark and flat-field corrections,
sky background subtraction, linearity correction, destripe and jitter
stacking. For more detailed description, we refer the reader to the
CASU webpage http://casu.ast.cam.ac.uk/surveys-projects/vista, as
well as to Irwin et al. (2004) and Lewis, Irwin & Bunclark (2010).
References: (1) Valenti & Fischer (2005); (2) van
Leeuwen (2007); (3) Caballero (2007).
2 I D E N T I F I C AT I O N A N D F O L L OW- U P
O B S E RVAT I O N S
2.1 VISTA Hemisphere Survey data
The new low-mass companion of HD 221356A was identified using the 2MASS and VHS catalogues. The VHS is a near-infrared
public survey intended to cover the entire Southern hemisphere
The search for additional common proper motion companions of
HD 221356A was done using the astrometry given in the VHS and
2MASS catalogues, which provide a 12.18-yr baseline. The positions of 2MASS sources have an estimated accuracy of 70–80 mas
over the magnitude range of 9 < K s ≤ 16 (Skrutskie et al. 2006). The
astrometric solution for VHS observations is done automatically as
part of the CASU pipeline, using the 2MASS point source catalogue. The objects on the catalogues extracted from each VISTA
detector are matched to their counterparts in 2MASS using a correlation radius of 1 arcsec. Because 2MASS has a high degree of
internal consistency it is possible to calibrate the world coordinate
system of VISTA images to better than 0.1 arcsec.
The search was performed using TOPCAT1 (Taylor 2005), a useful
tool for analysis and manipulation of source catalogues and other
data tables, developed as part of the Virtual Observatory. We retrieved the astrometric and photometric data from both 2MASS and
VHS catalogues, for all the objects within a radius of 15 arcmin corresponding to ∼23 000 au around the examined star. To avoid some
of the spurious detections, we selected sources brighter than J =
17 mag in 2MASS. We have cross-matched 300 objects from both
catalogues within 1 arcsec. The sources that remained unmatched
were subsequently cross-correlated taking into account the proper
motion of the primary star provided by Hipparcos (van Leeuwen
2007). We illustrate the resulting proper motion vector-point diagram of HD 221356 on Fig. 1.
We have found that the proper motion of HD 221356BC
(Table 2) is consistent with that of the primary HD 221356A,
thereby confirming the result of the Königstuhl survey of Caballero
(2007). Individual components of the BC pair, separated by only
0.57 arcsec (Close et al. 2002), were not resolved in the VHS images. We also identified a new common proper motion companion (2MASS J23313095−0405234, hereafter HD 221356D), with
(μα cos δ, μδ ) = (153.48 ± 21, −190.20 ± 19) mas yr−1 , located
12.13 ± 0.18 arcsec south-west from the primary. We adopted a total
astrometric uncertainty of ∼28 mas yr−1 , estimated using the standard deviation of proper motions for sources with μ < 100 mas yr−1 .
The proper motion of the new object is common with that of the HD
221356 system. Measured separations, position angles and proper
motions are listed in Table 2.
1
http://www.star.bris.ac.uk/∼mbt/topcat/
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2.2 Proper motion
We are conducting a search for very low mass common proper
motion companions of nearby (25 pc) stars, using the Visible and
Infrared Survey Telescope for Astronomy (VISTA) Hemisphere
Survey (VHS; McMahon et al., in preparation) and Two Micron All
Sky Survey (2MASS; Skrutskie et al. 2006). Our sample of objects
includes some of the known multiple systems (Faherty et al. 2010,
and references therein). One of the targets investigated so far was
HD 221356, already known to be a triple system. The F8 V primary
is a field star with slightly subsolar metallicity [Fe/H] = −0.26
(Valenti & Fischer 2005), located at 26.12 ± 0.37 pc (van Leeuwen
2007). The main properties of this star are given in Table 1. Gizis
et al. (2000) reported that the secondary, initially described as a
single object, has a photometric distance consistent with that of the
HD 221356 star determined by Hipparcos. The secondary was later
resolved by Close et al. (2002) into a binary separated by 0.57 arcsec
(∼14.9 au), using adaptive optics on the Gemini North Telescope.
Based on their photometric colours, they estimated spectral types
of M8 V and L3 V for each of the two components, respectively.
This binary (hereafter referred to as HD 221356BC) was also investigated by Caballero (2007). Using data at epochs separated by 48.3
years, he confirmed a common proper motion between HD 221356A
and HD 221356BC. He also measured a mean separation of ρ =
451.8 ± 0.4 arcsec between both components, which corresponds
to a projected physical separation of nearly 12 000 au, making it one
of the widest known systems with an L-type component (see also
fig. 11 of Zhang et al. 2010).
In this paper we present the identification and characterization
of a fourth, very low mass companion of the HD 221356 system.
We outline the procedure and results of our proper motion search
together with the analysis of I- and Y J H Ks -band photometry and
near-infrared spectroscopic data of the identified companion. We
derive the physical properties of the new object which turns out to
be very close to the hydrogen-burning mass limit.
A new L dwarf member of the HD 221356 system
Table 2. Proper motion, separations and position angles of low-mass components of the HD 221356 system.
Comp.
BC
D
μα cos (δ)
μδ
(mas yr−1 ) (mas yr−1 )
176 ± 21
154 ± 21
ρa
(arcsec)
θb
(◦ )
r
(au)
−167 ± 19 451.10 ± 0.18 261.77 ± 0.04 11900 ± 50
−190 ± 19 12.13 ± 0.18
221.8 ± 1.7
317 ± 9
(MJD) = 55525.12460836; ρ, θ and r are measured with respect to
the primary.
a Epoch
2.3 Photometry
The VHS catalogue provides aperture photometry for HD 221356
in the Y JHK s near-infrared bands. The VISTA photometric system is calibrated using the magnitudes of colour-selected 2MASS
stars converted on to the VISTA system using colour equations,
including terms to account for interstellar reddening.2 Photometric
calibrations are determined to an accuracy of 1–2 per cent.
The new faint companion HD 221356D is well resolved in the
VHS images (see Fig. 2), but within the glare of the primary. In
order to minimize the possible light contamination in the aperture
photometry of the VHS catalogue, we applied a method to suppress the point spread function (PSF) of the primary by subtracting
the flipped and rotated images from the original ones. Standard
deviation of the background at the same separation as the companion was a factor 2 lower in the PSF-subtracted images than in the
original ones, which slightly improves the detection of the object.
We performed aperture photometry on the resultant images with
an aperture of one full width at half-maximum (FWHM). The instrumental magnitudes were then calibrated to apparent magnitudes
by adding the aperture corrections determined using the VHS photometry of 14 isolated bright stars located within 10 arcmin from
HD 221356A. The differences between the new and the catalogue
photometry in the Y, J, H and K s bands are 0.53 ± 0.06, 0.26 ±
2
http://casu.ast.cam.ac.uk/surveys-projects/vista/technical/photometricproperties
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Monthly Notices of the Royal Astronomical Society Figure 2. False colour VISTA J-band image of HD 221356AD. Angular
separation is 12.13 ± 0.18 arcsec and the position angle of the identified
companion is 221.◦ 8 ± 1.◦ 7. Saturation in the centre of the primary is visible.
Field of view is 1 × 1 arcmin, with north up and east to the left.
0.04, 0.15 ± 0.03 and 0.13 ± 0.03 mag, respectively. Photometric
values for the BC companion were taken directly from the VHS
source catalogue.
On 2011 November 15, we performed follow-up I-band imaging
of HD 221356AD. Observations were carried out using the IAC80
telescope equipped with an E2V 2048 × 2048 back-illuminated
CCD detector with a plate scale of 0.304 arcsec pixel−1 , which provides a 10.4 × 10.4 arcmin field of view. We selected the 12 images
with best seeing (FWHM < 1.2 arcsec) to minimize interference
from the primary star. We reduced the images applying standard
techniques, including bias and flat-field correction, using IRAF routines. Individual exposure times were 5 s. For each image, we performed a similar method for the PSF subtraction of the primary as
used for VISTA images and subsequently aligned and combined
all of them. We obtained the PSF-fit photometry using the DAOPHOT
package in IRAF and calibrated the instrumental magnitude of our
object using 11 bright stars in the field with Deep Near Infrared
Survey of the Southern Sky (DENIS, Epchtein et al. 1999) I-band
data available. We note that the photometric system used (Cousin)
is not the same as that of DENIS, and that some differences may
appear for very cool objects; however, in our previous photometric calibrations we found that the zero-point between IAC80 and
DENIS I band has small colour dependence (Costado et al. 2005).
Additionally, we acquired I-band observations of HD 221356A
using FastCam, mounted on the 1.5-m Carlos Sánchez Telescope
at the Teide Observatory on 2012 January 31. FastCam is a lucky
imaging instrument, designed to perform high spatial and time resolution observations (Oscoz et al. 2008). Optics provide a plate scale
of 43.5 mas pixel−1 and a field of view of ∼22 × 22 arcsec2 . We
obtained 17 blocks of 1000 images of 50-ms individual exposure
times. Images were bias corrected, aligned and stacked into the final
image using the software provided by the FastCam team. We derived the I-band aperture photometry of the primary, since we found
that the literature values based on photographic plates are not reliable. Instrumental magnitudes were calibrated using photometric
standard stars from Landolt (1992) observed at different airmasses
along the night under photometric conditions. We also explored
the inner region to search for the presence of additional companions, but none was identified. We may exclude the presence of an
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Figure 1. Proper motion vector-point diagram for the HD 221356 system.
All correlated objects within 15 arcmin from the primary are plotted as black
dots, with HD 221356 components labelled as A, BC and D. The primary
is saturated in both surveys; its proper motion value was taken from the
literature. Time baseline between the 2MASS and VHS epochs is 12.18 yr.
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Table 3. Photometric data for the components of the multiple system HD 221356.
Component
I (mag)
Y (mag)
J (mag)
H (mag)
K s (mag)
A
BC
B
C
D
5.953 ± 0.018
15.536 ± 0.027
15.568 ± 0.018
19.363 ± 0.039
16.70 ± 0.10
–
13.695 ± 0.025
–
–
14.933 ± 0.059
5.488 ± 0.019
12.852 ± 0.010
12.933 ± 0.011
15.713 ± 0.042
13.763 ± 0.038
5.264 ± 0.038
12.261 ± 0.026
12.353 ± 0.026
14.993 ± 0.061
13.209 ± 0.026
5.150 ± 0.017
11.946 ± 0.026
12.055 ± 0.028
14.495 ± 0.114
12.755 ± 0.025
Note. I-band magnitude of A is from the FastCam, JHK s magnitudes are from 2MASS. I magnitude of the
BC is from Gizis et al. (2003), JHK s magnitudes are from VHS and were decomposed using the flux ratios
derived by Close et al. (2002). I magnitude of D is from IAC80 measurement, JHK s magnitudes are from our
photometry on VHS images.
2.4 Near-infrared spectroscopy
We also obtained near-infrared spectroscopy of the HD 221356AD
system and the M8V spectroscopic standard LP 412−31 (Kirkpatrick et al. 1995) using the Long-Slit Infrared Imager and Spectrograph (LIRIS), with the HK grism and the 1K × 1K Hawaii detector
at the 4.2-m William Herschel Telescope (WHT) on 2011 December
30. This instrumental configuration provides a nominal dispersion
of 9.7 Å pixel−1 and a wavelength coverage of 1.4–2.4 µm. A slit
width of 0.75 arcsec was used rotated to the direction along the AD
system and the final resolution of the spectrum was 26 Å (R ∼ 600).
Total integration time was 2240 s, divided into individual exposures
of 160 s. A nodding pattern of two positions (AB) was used to subtract the sky background. Weather conditions were photometric and
the average seeing was 0.9 arcsec. Data were dark corrected, sky
subtracted, aligned and combined at each nodding position. Flatfield correction using a tungsten lamp was not applied due to a
spurious feature in the K-band tungsten spectrum. After subtracting the contribution of the primary wings, spectra of HD 221356D
were optimally extracted using the APALL routine and wavelength
calibrated using ArXe arc lines. We finally combined the spectra
at both AB positions and corrected for telluric absorption lines by
dividing them by the A3V star HR 8840, observed at a similar airmass, and multiplying by a blackbody of the corresponding effective
temperature of 8500 K. Spectroscopic data of LP 412−31 were reduced and analysed in a similar way to HD 221356D, but telluric
correction was done using the A3V star HR 1036, also observed
at a similar airmass. Spectra of HD 221356D and LP 412−31 in
comparison with other standard objects and the main spectroscopic
features are shown in Fig. 3.
HD 221356D displays stronger water vapour absorption bands
than the standard M8 dwarf observed with the same instrumental
set-up and sky conditions and reduced in the same manner as our
target. This implies that HD 221356D has a cooler spectral type,
quite likely within the L domain. Aimed at deriving the spectral
type of HD 221356D, we have compared its LIRIS spectrum with
data extracted from the Infrared Telescope Facility (IRTF) libraries
(Cushing et al. 2005) – see Fig. 3. We note that these data correspond
mostly to dwarfs with solar composition. The overall HK spectral
energy distribution (SED) of HD 221356D is better reproduced by
a spectral type of L0–L1. However, we determine a spectral type
of L1–L3 if we consider the different water indices at ∼1.7 or
2.0 µm (Testi et al. 2001; Slesnick, Hillenbrand & Carpenter 2004;
Allers et al. 2007). The differences can be explained if the object
is slightly metal poor (consistent with the primary), because the Kband flux should be reduced by the H2 collision-induced absorption.
Figure 3. Near-infrared H- and K s -band spectra of the new companion compared with the M8 standard LP 412−31 (Kirkpatrick, Henry & Simons 1995),
observed with the same instrumentation, and the L1 (2MASS J14392836+1929149) and L3 templates (2MASS J15065441+1321060) taken from the IRTF
library (Cushing, Rayner & Vacca 2005). Spectra were normalized at 1.7 µm and offsets have been added for clarity. The grey area indicates the region of high
telluric absorption. The most prominent molecular and atomic features are indicated.
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equal-mass companion to the primary at separations greater than
0.2 arcsec (∼5 au) and companions with I < 5 mag at separations
greater than 1 arcsec (∼26 au).
The photometric data are listed in Table 3. The I-band magnitudes of the distant BC pair were taken from Gizis et al. (2003) who
imaged individual components with Hubble Space Telescope Wide
Field Planetary Camera 2 (WFPC2). The integrated JHK s photometry of the BC component from VHS was also decomposed into
individual magnitudes using the flux ratios derived by Close et al.
(2002). The JHK s -band photometry of the primary, also given in
Table 3, is from the 2MASS catalogue.
A new L dwarf member of the HD 221356 system
The strength of the sodium feature at 2.2 µm in object D compares
better with solar metallicity M9 dwarf than with the early L-type
templates; however, we prefer to rely on the general SED, which
is better fitted by L1 templates, than on a single feature that can
be uncertain bearing in mind the poorly understood effects of low
metallicity.
We also note that the water vapour absorption band at ∼1.5 µm,
in the blue part of the H band is more intense in this object than
in early L dwarfs and more resembles mid/late L dwarfs. This is
not an instrumental effect since it does not appear in the M8.5
object, but it may be due to a larger contamination by the primary
at blue wavelengths than at red wavelengths. We therefore cannot,
with confidence, assign this feature as unusual for this object. In
summary, we estimate that HD 221356D is a slightly blue early L
dwarf with a spectral type of L0–L2.
3 P H Y S I C A L P RO P E RT I E S O F H D 2 2 1 3 5 6 D
and T dwarfs from Vrba et al. (2004) and Kirkpatrick et al. (2011),
and 12 subdwarfs with spectral types from M7 to L7 with measured
parallaxes (Faherty et al. 2012). In the (M I , I − J) diagram, we have
also plotted the 5-Gyr isochrones of the NextGen models for solar
and low metallicity ([M/H] = −0.5) (Baraffe et al. 1998) and of the
DUSTY model for solar metallicity (Chabrier et al. 2000).
We have depicted a variety of objects with similar spectral types
that span from young ages (∼120 Myr) and solar composition to old
stars with solar metallicities and field subdwarfs with metal-poor
abundances ([Fe/H] ∼ −0.5; Lépine, Rich & Shara 2007). The lowmass components of the HD 221356 system have slightly bluer
colours than typical field stars, placing them on the blue edge of the
photometric sequence defined by late M and L dwarfs. However,
their colours are redder than known field subdwarfs, implying an
intermediate metallicity, being thus in good agreement with the
metallicity determination of the primary. The J − K s and I − J
colours of the new companion, which are 1.01 ± 0.06 and 2.94 ±
0.14 mag, respectively, correspond to the typical values of M7–
M9 field dwarfs (Kirkpatrick & McCarthy 1994; Leggett et al.
2002). These photometric colours suggest an earlier spectral type
than that determined in the spectroscopic analysis. We attribute this
difference to the subsolar metallicity of the system, which affects
the SED. In particular, the flux suppression in the K s band was
already recognized in a group of L dwarfs, as a low-metallicity
feature (Kirkpatrick et al. 2010; West et al. 2011). Because of that,
the spectral classification of the wide binary components of the HD
221356 system, which is based on photometry, may be uncertain.
The luminosities of the B, C and D components were derived from
their JHK s -band magnitudes, using the trigonometric distance of the
primary, bolometric corrections from Golimowski et al. (2004) and
spectral type–colour relations from Vrba et al. (2004). The effective
temperature ranges were calculated adopting the temperature scale
Figure 4. Left-hand panel: M I , I − J colour–magnitude diagram of the HD 221356 system. The positions of the four components are marked with points and
labelled with the corresponding letters. Pleiades low-mass stars and brown dwarfs (crosses) from Bihain et al. (2010), M and L dwarfs from Liebert & Gizis
(2006) with available parallaxes and M dwarfs (dots) from Leggett et al. (2000) are also plotted. The 5-Gyr isochrones of the NextGen models for solar and
low-metallicity stars (Baraffe et al. 1998), represented by a dotted line and a dashed line, respectively, and of the DUSTY model for solar metallicity (Chabrier
et al. 2000), shown as a solid line, are also included. The masses in solar mass units from the DUSTY model are also indicated and marked with open circles in
the corresponding isochrone. Right-hand panel: M J , J − K of BCD components of the HD 221356 system. We have also added L and T dwarfs from Vrba
et al. (2004) and Kirkpatrick, Cushing & Gelino (2011), and 12 subdwarfs with measured parallaxes from Faherty et al. (2012), depicted by squares. The mean
L and T near-infrared photometric sequence from Vrba et al. (2004) is represented by a dashed line.
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Assuming a distance of 26.12 ± 0.37 pc to the system, we calculated
the absolute magnitudes of individual components and constructed
the (M I , I − J) and (M J , J − K s ) colour–magnitude diagrams, shown
in Fig. 4. All four objects clearly follow a well-defined photometric
sequence, with the new companion located between the known B
and C components. All the colours and magnitudes of the new
object are in good agreement with its physical membership to the
HD 221356 system. To better illustrate the position of the three lowmass companions, we added in both panels of Fig. 4 the Pleiades
low-mass stars and brown dwarfs (at d = 120 pc) from Bihain et al.
(2010), M and L dwarfs from Liebert & Gizis (2006) with available
parallaxes and the field M dwarfs from Leggett et al. (2000). The
Pleiades cluster offers a homogeneous collection of objects with
similar age and metallicity. The right-hand panel also includes L
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Table 4. Physical properties of low-mass companions in HD 221356
system.
Comp.
Sp. type
log L/L
T eff (K)
M/M
B
C
D
M8.0 ± 1.5
L3.0 ± 1.5
L1.0 ± 1.0
−3.28 ± 0.09
−4.35 ± 0.09
−3.61 ± 0.10
2351–2717
1756–2141
2079–2304
0.090 ± 0.008
0.065+0.007
−0.016
0.079 ± 0.006
4 A B E N C H M A R K S U B S O L A R M E TA L L I C I T Y
M U LT I P L E S Y S T E M
The age of the primary star in the HD 221356 multiple system
was estimated by Valenti & Fischer (2005) to be 2.5–7.9 Gyr based
on isochrone analysis. The lithium abundance of the primary star
[log n(Li) = 2.5 in the usual scale of log n(H) = 12] is typical of
late F-type stars in clusters with ages in the 2–8 Gyr range (Sestito
& Randich 2005). The chromospheric activity of the star is also
typical of a moderately old main-sequence star (Valenti & Fischer
2005).
HD 221356 is a slightly metal poor stellar system whose components have masses just above and below the hydrogen-burning
limit. For subsolar metallicities the stellar–brown dwarf borderline
is expected to be shifted to higher masses, e.g. to ∼0.079 M at
[M/H] = −0.5 (0.072 M at [M/H] = 0) (Baraffe et al. 1998). In
such a coeval, old system, it becomes particularly interesting to investigate the lithium abundances of the very low mass components.
While the M8 star (component B) and the spectral type range L0–
L2 (component D) should have fully burnt their original lithium,
component C with 0.065 M may have preserved some amount of
the initial lithium content. Theoretical models for solar metallicity
predict full lithium depletion for such a mass; however, this may not
be the case for subsolar metallicity, since models also predict a less
efficient depletion at low metallicities (Chabrier & Baraffe 1997).
Observations of the Li abundance in the three low-mass components
of this system will constrain both the evolutionary models and the
age of the system.
Multi-epoch measurements of the system will allow us to detect
the orbital motion of companion D. Although the estimated orbital
period (assuming circular orbit) is of the order of 5500 years, the
relative change of the position would be up to ∼14 mas yr−1 , which
5 CONCLUSIONS
Using the VHS and 2MASS surveys we have identified a new very
low mass companion (HD 221356D) in the slightly metal poor HD
221356 system, which thus becomes a quadruple. The new object
is located at a projected distance of ∼312 au from the F8 primary.
The four components of the system follow a well-defined photometric sequence. From near-infrared spectroscopy we determined
L0–L2 spectral type for the D companion. Based on evolutionary
models its mass is estimated at 0.079 ± 0.006 M , and its effective temperature is in the range 2100–2300 K. The J − K s and
I − J colours of the low-mass components are slightly bluer than
field counterparts of the same spectral type. We interpret this as a
result of the low metallicity of the system, which may become a
reference for the spectral classification of metal-poor M and L-type
field objects. Since the distance and metallicity of the HD 221356
system are well known, the detailed study of its ultracool companions, which are located above and below the frontier between stars
and brown dwarfs, can provide valuable constrains on evolutionary
models and, in particular, shed light on the properties of objects on
the transition from stellar to substellar regime.
AC K N OW L E D G M E N T S
Based on observations obtained as part of the VISTA Hemisphere
Survey, ESO Programme: 179.A-2010 (PI: McMahon). The VISTA
Data Flow System pipeline processing and science archive are described in Irwin et al. (2004) and Cross et al. (2009). This paper
is based on observations made with the TCS and IAC80 telescope
operated on the island of Tenerife by the IAC in the Spanish Observatorio del Teide, and with the William Herschel 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 Astrofı́sica de Canarias. This publication makes use of
data products from the Two Micron All Sky Survey, which is a
joint project of the University of Massachusetts and the Infrared
Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration
and the National Science Foundation. This research has benefitted
from the SpeX Prism Spectral Libraries, maintained by Adam Burgasser at http://pono.ucsd.edu/adam/browndwarfs/spexprism. This
research has been supported by the Spanish Ministry of Economics
and Competitiveness under the projects AYA2010-21308-C3-02,
AYA2010-21308-C03-03 and AYA2010-20535. VJSB and NL are
partially supported by the Spanish Ramón y Cajal programme.
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