Issue |
A&A
Volume 514, May 2010
|
|
---|---|---|
Article Number | A18 | |
Number of page(s) | 10 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/200913940 | |
Published online | 04 May 2010 |
Stars and brown dwarfs in the
Orionis
cluster
II. A proper motion study
J. A. Caballero
Departamento de Astrofísica y Ciencias de la Atmósfera, Facultad de Física, Universidad Complutense de Madrid, 28040 Madrid, Spain
Received 22 December 2009 / Accepted 20 January 2010
Abstract
Aims. I attempt to fully understand the origin of
the stellar and substellar populations in the young Orionis
open cluster, which is a benchmark for star-forming studies. Because of
the very low proper motion of the cluster, late-type dwarfs with
appreciable proper motion in the foreground of
Orionis can be
easily discarded as targets from expensive spectroscopic follow-up
studies.
Methods. I use the Aladin sky atlas, USNO-B1, public
astrometric catalogues, and photographic plate digitisations to
identify stars with proper motions that are inconsistent with cluster
membership in a circular area of radius 30 arcmin centred on
the early-type multiple system Ori. Primarily
because of the long time baseline of more than half a century, the
errors in the measured proper motions are lower than
2 mas a-1.
Results. Of the 42 stars selected for astrometric
follow-up, 37 of them are proper-motion cluster interlopers. Some
USNO-B1 measurements were affected by partially resolved (visual)
multiplicity and target faintness. Because of their late spectral types
and, hence, red colours, 24 contaminants had been considered at some
point as Orionis
members. I discuss how contamination may have affected
previous work (especially related to disc frequencies) and the curious
presence of lithium absorption in three M-dwarf proper motion
contaminants. Finally, I classify the bright star
HD 294297 as a late-F field dwarf unrelated to the cluster
based on a new proper motion measurement.
Conclusions. Although proper motions cannot be used
to confirm membership in Orionis,
they can be instead used to discard a number of cluster member
candidates without spectroscopy.
Key words: astronomical databases:
miscellaneous - stars:
late-type - open clusters and associations: individual:
Orionis
- proper motions
1 Introduction
The Orionis
cluster (
Ma,
pc)
in
the Ori OB 1b association is a unique site
for investigating the
formation and evolution of stars and substellar objects from several
tens of
solar masses to a few Jupiter masses (Garrison 1967; Wolk 1996; Béjar
et al. 1999;
Caballero 2007a;
Walter et al. 2008,
and references therein). Although it is not as young and nearby as
other star-forming regions such as the
Orion Nebula Cluster,
Ophiuchi, or
Chamaeleon I+II,
Orionis has the
great advantage of being
relatively compact (
arcmin
- Caballero 2008a)
and
having a very low visual extinction (0.04 mag
< E(B-V)
< 0.09 mag - Béjar
et al. 2004;
Sherry et al. 2008).
The high spatial density and low extinction do not only facilitate the
study of
the initial mass function down to well below the deuterium burning
limit
(Zapatero Osorio et al. 2000; Béjar
et al. 2001;
Caballero et al. 2007;
Bihain et al. 2009),
but also the identification and characterisation of young
stars with X-ray emission (Sanz-Forcada et al. 2004; Franciosini
et al. 2006;
Skinner et al. 2008;
Caballero et al. 2009)
or discs detected on the basis of their infrared excess (Oliveira
et al. 2006;
Hernández et al. 2007;
Zapatero Osorio et al. 2007; Luhman
et al. 2008),
hydrogen recombination lines in emission (including H
- Zapatero Osorio
et al. 2002;
Weaver & Babcock 2004;
Kenyon et al. 2005;
Caballero et al. 2006;
Gatti et al. 2008;
Fedele et al. 2010),
or jets (Reipurth et al. 1998; Andrews
et al. 2004).
Most of the works listed above compute frequencies of X-ray
emitters, Haccretors,
or disc hosts, some of which are extensively used in the
literature (e.g., Hernández et al. 2007). Accurate
frequency determinations require a precise knowledge of the
Orionis
stellar and substellar populations. A large fraction of the cluster
member candidates, especially in the inner
20 arcmin, are known to have features of extreme youth (e.g.,
very early
spectral types, Li I
6707.8 Å
in absorption, H
in
strong broad emission, flux excess in the near- and mid-infrared,
spectroscopic
signatures of low gravity - Caballero 2008c).
However, there is evidence that there is significant contamination by
field
dwarfs (Caballero et al. 2008a;
Lodieu et al. 2009),
apart from overlapping
young stellar populations of the Orion Belt (e.g.,
Jeffries et al. 2006;
Caballero 2007a;
Sacco et al. 2007;
González-Hernández et al. 2008) and
galaxies (Caballero et al. 2007, 2008b;
Hernández et al. 2007;
Caballero
2008c).
The usual procedure for identifying a true Orionis
member is the
spectroscopic follow-up after photometric selection in a
colour-magnitude
diagram.
If no spectroscopy is available and the combination of photometric band
passes
is not optimal, then a large number of (foreground) field dwarfs may
contaminate
a sample.
One way of maximising the telescope time devoted to the spectroscopic
follow-up
of reliable cluster member candidates is to discard beforehand those
that have
proper motions inconsistent with cluster membership.
In
Orionis,
this method of de-contamination has been applied in only a
few occasions (Caballero 2007a,
2008c;
Lodieu et al. 2009)
. Depending on the accuracy
of the data used, stars with proper motions larger
than 10 (Hipparcos and Tycho-2), 20
(USNO-B1), and 30 mas a-1(2MASS/UKIDSS
Galactic Cluster Survey) were classified as foreground field
stars. These relatively small values are due to the location of
Orionis
close
to the antapex, which combined with a distance of almost
400 pc lead to a
proper motion of the cluster centre of mass of only (
,
,
) mas a-1
(Caballero
2007a);
this proper motion is consistent with the value provided by Kharchenko
et al. (2005).
In this work, I use Virtual Observatory tools and
data archives to search for
and characterise field stars in the direction of Orionis with
relatively large proper motions, measured with accuracies superior to
2 mas a-1. Most of the
interloper stars seem to be of late spectral types. Because of the
resemblance between their magnitudes and colours and those of
young late-type stars in
Orionis,
an important fraction of the
proper-motion interlopers had actually been selected as cluster member
candidates in photometric surveys (Scholz & Eislöffel 2004; Sherry
et al.
2004;
Kenyon et al. 2005;
Caballero 2006;
Hernández et al. 2007).
2 Analysis
2.1 Aladin search
I used the stellar proper motions tabulated by the United States Naval
Observatory USNO-B1 catalogue (Monet et al. 2003), which has
a wider coverage of
target magnitudes and is less affected by unresolved binarity than the
most
recent Positions and Proper Motions-Extended catalogue (PPMX - Röser
et al. 2008).
The Lick Northern Proper Motion 2 catalogue (LNPM2 - Hanson
et al. 2004),
which
also covers the Orion region, is severely affected by
systematics.
First, I used the Aladin sky atlas (Bonnarel et al. 2000) to
cross-match the
USNO-B1 and Two-Micron All Sky Survey (2MASS - Skrutskie
et al. 2006)
catalogues in a circle area of radius arcmin centred on
the
Trapezium-like multiple stellar system
Ori (Fig. 1),
which is located at the centre of the
Orionis cluster
(Caballero
2008b).
A total of 5421 USNO-B1 sources have a 2MASS near-infrared counterpart
within
4 arcsec (in practice, I looked for the USNO-B1
counterparts of 2MASS sources).
By cross-matching the two catalogues, it was possible to filter most of
the
numerous spurious USNO-B1 sources found in fields with high background
and
bright stars, as in this case.
Next, I used the Aladin tool VOplot to select USNO-B1/2MASS
stars with four or
five detections in the USNO-B1 catalogue (
), J-band
magnitudes brighter than 15.5 mag, and proper motions
mas a-1.
Very faint sources or those with detections at only two or three
astrometric
epochs (of a maximum of five epochs) have relatively large errors in
proper
motion and did not pass the previous filter. Of the 5421 USNO-B1/2MASS
sources, only 42 (0.8%) objects satisfied my
selection criterion.
They are shown in Table A.1.
The USNO-B1 proper motions of the 42 objects vary between 40
and
290 mas a-1, with 31 having
detections at five epochs.
![]() |
Figure 1:
Inverse-colour digitised image of the red plate (RJ)
of the Digital
Sky Survey II centred on the Trapezium-like |
Open with DEXTER |
I chose the 40 mas a-1 limit to take into account:
- the increasing dispersion in intrinsic velocity, which is
proportional to
the dispersion in the proper motion, with decreasing masses. In a
sample of bona fide brown dwarfs of the
well-known Pleiades cluster,
Bihain et al. (2006)
measured an intrinsic velocity dispersion at least four
times larger than for cluster stars with masses of the order or higher
than
1
, as expected for a nearly relaxed cluster. Since limit values of 10 mas a-1 had been adopted for the most massive stars in
Orionis (Caballero 2007a), a limit value four times larger naturally arises for low-mass cluster stars (although the
Orionis cluster is not relaxed - Caballero 2008b);
- the increasing number of proper-motion interloper candidates to be followed-up for decreasing limit values. While there were 42 objects to be followed-up in the case of the 40 mas a-1 limit, there were 77, 161, and 492 such objects for the 30, 20, and 10 mas a-1 limits, respectively. Going below the 40 mas a-1 limit would make the target sample unmanageable;
- and the increasing number of USNO-B1 sources with spurious, incorrect, tabulated values for decreasing proper motions.
2.2 Astrometric follow-up
I followed the same methodology as Caballero (2009) in measuring high accuracy proper motions for the 42 objects. Basically, I used public data from digitised photographic plates and astro-photometric catalogues of the last half century. I gave special emphasis to the use of data from USNO-A2 (Monet et al. 1998), Guide Star Catalogues 2.2 and 2.3 (GSC2.2 and GSC2.3 - STScI 2001, 2006), 2MASS, Deep Near Infrared Survey of the Southern Sky (DENIS - Epchtein et al. 1997), Carlsberg Meridian Catalog 14 (CMC14 - Evans et al. 2002; Muiños 2006), UKIRT Infrared Deep Sky Survey (UKIDSS - Lawrence et al. 2007), and the SuperCOSMOS digitisations of plates from the United Kingdom Schmidt Telescope (UKST) and the first epoch of the Palomar Observatory Sky Survey (POSSI - Hambly et al. 2001).
The relatively poor astrometric precision of the plate
digitisations, of
0.4-0.5 arcsec, was counterbalanced by the long interval
between epochs.
In Table 1,
I show an example of the nine astrometric
epochs used to compute the proper motion of star No. 24
(G 99-20).
In this case, almost 54 years passed between the
first and last epochs.
For the other 41 objects, a typical number of astrometric
epochs was also eight or nine (
;
with maxima and minima of ten and seven epochs, respectively), and they
also covered about 54 years. Except for the CMC14
epochs, which have an accuracy of one day, all the
tabulated astrometric epochs have a temporal precision better than
0.0001 modified Julian days. Depending on when the
corresponding field was surveyed, the epochs for each
dataset varied from one target to other, but remained within a narrow
interval
(e.g., all 2MASS measurements in the
Orionis area were
taken between
J1998 and J2000).
Only the epoch for the the POSSI Red (USNO-A2.0) measurement,
J1951.908,
remained identical for all the sources in the area.
Table 1: Astrometric epochs for star No. 24 (G 99-20).
When there was no detection by GSC2.2/2.3 and/or USNO-A2.0
(especially because
of target faintness), I used instead the red optical
digitisations of POSSI and
UKST. Blue and infrared UKST digitisations were used in all case. Star
centroid positions were measured with the IRAF environment using
standard
tasks. Occasionally, there was an additional, intermediate, astrometric
epoch at J1984
from GSC1.2/ACT (Lasker et al. 1988), or double
DENIS detections at different epochs between J1995 and J1999.
Apart from this, the faintest objects had in general no CMC14
measurements.
Of the several UKIDSS unmerged astrometric data sets,
I considered those taken
in the K band, which are of higher spatial
resolution than to those taken in
the Y, Z, J,
and H bands.
The astrometric accuracy for each epoch ranged between
0.06-0.07 arcsec
for 2MASS (when tabulated, CMC14 accuracy was even higher) to 0.4-0.5
for
USNO-A2.0 and GSC2.2/2.3 or point spread fittings of red targets in the
blue
UKST images.
Proper motions in right ascension
and declination
were computed independently using simple linear fits with time
(i.e., possible contribution by parallax was not accounted for).
Errors were estimated from the standard deviations of the differences
between
the observed values
and
and the expected values
and
from the linear regression.
These errors in proper motion were always smaller than
2 mas a-1, with
average proper motion errors of only 0.92 and
0.97 mas a-1 in right
ascension and declination, respectively, which are comparable to the
Tycho-2 catalogue errors (Høg et al. 2000).
However, the objects studied for this paper are much fainter: only nine
of the
42 objects are brighter than J = 12 mag,
while three objects are fainter than
J = 15 mag.
Because of their red colours, many targets are close to the limiting
magnitude
of the blue and red optical photographic plates (
mag). As examples,
Figs. 2
and 3
show the
temporal variation in the star coordinates and the fit for the high
proper-motion star No. 24 (G 99-20).
The proper motions and number of epochs used in preparing the astrometric follow-up of the 42 objects are given in the last columns of Table A.1.
![]() |
Figure 2:
Inverse-colour SuperCOSMOS digitised images of the red plates (RJ)
of POSSI (J1951.9, left) and UKST (J1991.0,
right) centred on the
J2000 position of star No. 24 (G 99-20). Size is
5 |
Open with DEXTER |
![]() |
Figure 3: Right ascension and declination as a function of modified Julian date for the star No. 24 (G 99-20). Dashed (red) lines indicate the linear fits. The largest errorbars correspond to the UKST Blue (GSC2.2/2.3) astrometric epoch. |
Open with DEXTER |
3 Results
3.1 Objects with incorrect USNO-B1 proper motions
The correspondence between the USNO-B1 proper motions and those measured in this work are in general acceptable, with variations smaller than about 10 mas a-1, but a number of noticeable exceptions exist. There are 15 investigated objects whose true proper motions differ from those inferred by USNO-B1 by 30-120 mas a-1. They were affected by visual multiplicity and faintness in the visible. The seven cross-matched sources with separations between the USNO-B1 and 2MASS counterparts of more than 0.9 arcsec (and up to 3.5 arcsec) are among those exceptions (the separations for the other 35 objects are of 0.6 arcsec or less).
The remaining 27 stars have reliable USNO-B1 proper motions
larger than mas a-1.
Of these, six are presented here for the first time (Nos. 01,
03, 19, 25,
28, and 35) and have J-band magnitudes and
proper motions in the approximate
intervals 40-60 mas a-1 and
13-14 mag, respectively, while the other
21 stars were investigated in previous work with more or less detail.
3.1.1 Visual multiple systems
Ten investigated stars with incorrect USNO-B1 proper motions have
visual
companions of roughly the same optical magnitudes at separations arcsec,
which were incorrectly resolved by at least one USNO-B1
astrometric epoch observation. Some properties of the visual companions
to the ten stars are shown in
Table 2
(stars Nos. 02 and 04 are in fact triple and
quadruple systems, respectively). None have proper motions larger than
20 mas a-1 or consistent
with membership of a physical system, and only one has ever been
considered as a
cluster member candidate (the visual companion to star No. 22;
see below).
Three of the ten main targets in visual multiple systems have
actual total
proper motions larger than 70 mas a-1
(Nos. 02, 12, and 18) and are,
therefore, high proper-motion interloper stars towards the
Orionis
cluster. The same is true of another four stars with proper motions
mas a-1
(Nos. 09, 20, 21, and 22). The remaining three stars
(Nos. 04, 38, and 42) have lower proper motions,
of 7-12 mas a-1,
which are consistent with cluster membership.
However, only one of them has ever been considered as a cluster member
based on
photometry. This star, No. 42 (Mayrit 1610344), was
first identified as a (0.39-
,
non-variable) photometric
Orionis
member candidate by Scholz &
Eislöffel (2004). Afterwards, Sherry et al. (2004) and
Caballero (2008c)
agreed on this
classification (although the first authors assigned a membership
probability of
only 15%), which should be adhered to until moderate-resolution visible
spectroscopy is obtained. The other two low proper-motion stars
(Nos. 04 and 38) have
visible/near-infrared (DENIS/2MASS) colours that are inconsistent with
cluster
membership for their magnitudes.
Table 2: Companions in visual binary and multiple systems with incorrect USNO-B1 proper motions.
Among the seven high proper-motion star contaminants in visual
multiple systems,
three targets merit additional descriptions.
No. 18.
This was first classified as a


No. 21.
This star has been cited only once, by Hernández et al. (2007), who classified it as a ``class III'' young object. However, star No. 21, with a proper motion
No. 22.
The system is formed by two stars equally separated from an X-ray source detected with XMM-Newton by López-Santiago & Caballero (2008). On the one hand, the brightest star in the pair, which has a low proper motion, has too blue DENIS/2MASS colours for its magnitude, inconsistent with cluster membership. On the other hand, the faintest star (Table 2), which was also considered to be a cluster member candidate by Sherry et al. (2004) and Hernández et al. (2007), has a proper motion of



3.1.2 Single objects
Discarding the ten previously considered stars in visual multiple
systems, there
are another five objects with proper motion differences between USNO-B1
and my
astrometric follow-up of mas a-1.
These variations are ascribed to target faintness
(Nos. 08, 23, and 32; the three
of them have
mag)
and high background in the visible (Nos. 29
and 36; both of them have a large separation from the cluster
centre and are
embedded in the IC 434 nebula, which is associated
with the
Horsehead Nebula - Fig. 1). Three of the
stars have both colours and magnitudes that are inconsistent with
cluster membership and have never been considered before in photometric
surveys.
However, the other two objects, which have variations of
mas a-1,
deserve attention.
No. 32 (Mayrit 999306).
The faint brown dwarf candidate Mayrit 999306
([BZR99] S Ori 23) was discovered
by Béjar et al. (1999)
and has since been identified in several other
independent photometric surveys (Béjar et al. 2001;
González-García
et al. 2006;
Caballero 2008c).
Its USNO-B1 proper motion, which was based on four astrometric epochs,
was severely affected by its extreme faintness in the blue optical:
while
Mayrit 999306 is relatively bright at 2.2 m (
mag),
it has photographic magnitudes
mag.
Its true proper motion, of about 14 mas a-1
is slightly large, but
still consistent with cluster membership.
Mayrit 999306 still awaits spectroscopic follow-up of moderate
spectral
resolution.
No. 36 (Mayrit 1493050).
This young star was discovered by Scholz & Eislöffel (2004). Afterwards, Kenyon et al. (2005) found lithium in strong absorbtion and low gravity features in its spectrum, while Hernández et al. (2007) claimed that it harbours an ``evolved disc'' based on IRAC and MIPS Spitzer photometry. Maxted et al. (2008) monitored its radial velocity, which is consistent with membership and with SB2 binarity. Mayrit 1493050 is located close to an overdensity in the IC 434 nebula, which may lead to an incorrect USNO-B1 measurement, possibly in the RF plates, where the nebula is brighter (because of the H

3.2 Cluster membership and non-membership
![]() |
Figure 4:
Proper motion diagram of the 42 main targets.
Typical errorbars are of the same size as the small (blue) dots.
The (red) circle and small square close to the origin of coordinates
indicate |
Open with DEXTER |
Table A.2
summarises the membership status of the 42
main targets (see also Fig. 4). Twenty of
them were photometric cluster member candidates in the works by
Scholz & Eislöffel (2004;
[SE2004]), Sherry et al. (2004;
[SWW2004]), and
Hernández et al. (2007;
[HHM2007]).
However, only three objects (No. 32/Mayrit 999306,
No. 36/Mayrit 1493050 - with
known spectroscopic youth features -, and
No. 42/Mayrit 1610344) remain as true
cluster member candiates.
The other 17 cluster member candidates do not belong to Orionis
based
on proper motion measurements and, in some cases, high-quality
spectroscopic
information from Kenyon et al. (2005), Caballero
(2006),
and Sacco et al.
(2008).
Of the other 22 stars, 15 had not been investigated in the
literature and seven
had been discarded as cluster members by Kenyon et al. (2005) and
Caballero
(2006, 2008c) based
on spectroscopy and/or proper motion.
To sum up, I identify for the first time 21
proper-motion interlopers and
contaminants towards the Orionis
cluster, of which eight were
previously considered as cluster member candidates based on photometry
(indicated by ``Yes?'' in the ``Previous member'' column of
Table A.2).
Another two stars with low actual proper motion, Nos. 04
and 38, have magnitudes
and colours inconsistent with cluster membership (Sect. 3.1.1).
In addition, I confirm the non-cluster membership of 16
foreground stars, of
which ten had never been discussed before in the literature.
The data for the other six stars with proper motion measurements were
tabulated
by Giclas et al. (1961;
No. 24), Caballero (2006;
Nos. 15 and 30), and Caballero
(2008c;
Nos. 05, 14, and 26).
Finally, of the three objects with low proper motions and
suitable magnitudes
and colours for Orionis
membership, two (Nos. 32 and 42, see above)
lack spectroscopy and should be followed-up in a future spectroscopic
study.
The third star, No. 36, is the only bona fide
member of cluster.
4 Discussion
4.1 Comparison to other works
Of the ten bright Tycho-2 (and Hipparcos)
proper-motion contaminants
towards Orionis
identified by Caballero (2007a),
only two had
mas a-1
(HD 294269, a G0 star with lithium abundance and
radial velocity inconsistent with cluster membership, and
TYC 4770 924 1)
. In spite of the two stars
being cross-matched with counterparts in the
USNO-B1/2MASS search (Sect. 2.1),
having similar
Tycho-2 and USNO-B1 proper motions, and being detected in five USNO-B1
astrometric epochs, they did not pass the VOplot filter. It is likely
that Tycho-2 stars, with mean epochs of observation 2000.0, are
handled by Aladin/VOplot differently from fainter stars with USNO-B1
proper
motions measured from the digitised plates (they have mean epochs of
observation 1970-1980). In any case, the work of Caballero (2007a) was
devoted to the brightest stars of
Orionis
and is complementary to the study presented here.
The three new stars with mas a-1
in the Mayrit catalogue
(Caballero 2008c)
also appear here (Nos. 05, 14, and 26).
The other four non-Tycho-2 proper-motion contaminants without
spectroscopy and
mas a-1
in the Mayrit catalogue did not pass the
VOplot filter.
Lodieu et al. (2009) tabulated eight stars with proper motions larger than 30 mas a-1, of which only one, No. 16 (with spectroscopic information in Sacco et al. 2008), is presented here as a proper-motion interloper. The other seven stars are:
- S Ori 20, a faint field
M5.5 dwarf with
and with previously insufficient membership information (Barrado y Navascués et al. 2003; Kenyon et al. 2005; Caballero et al. 2007),
- three stars of proper motion
mas a-1 (of which one was a photometric cluster member candidate in Sherry et al. 2004), and
- the young binary stars
Mayrit 92149 AB (for which Lodieu
et al.
2009
tabulated the two components) and
Mayrit 1106058 AB (the X-ray
star 2E 1486).
The two of them have true null proper motions: spurious values of
almost
400 mas a-1 resulted from
cross-matching the (unresolved) 2MASS and
(resolved) UKIDSS coordinates
. The two stars were first presented as binaries by Caballero (2007b) and Caballero (2008c), respectively, and exhibit incontrovertible indicators of youth.
4.2 Interloper M dwarfs with lithium?
Following Kenyon et al. (2005), ``factors of 100 depletion in lithium are probably needed to reduce'' values of Li I pseudo-equivalent widths, pEW(Li I)s, to about 300 mÅ or weaker in the spectra of late-type objects. This is because the lithium is burned in the fully convective interiors of low-mass pre-main sequence stars during the first 20-140 Ma. However, the typical pEW(Li I) of T Tauri stars in

Three of the proper-motion contaminants in this work have high
signal-to-noise
ratio spectroscopy around H
and Li I, taken by Sacco
et al. (2008)
with FLAMES at the VLT.
They are the stars Nos. 11, 16, and 18, which have
radial velocities and
pEW(Li I)s
(and H
emission?) inconsistent with membership in
Orionis
(Table 3).
Their proper motions vary between 60 and 100 mas a-1,
which are
values much greater than expected for any cluster member.
However, in spite of their late spectral type (M1.5-3.0), the spectra
of these
field dwarfs apparently display lithium in
absorption, as measured with
exquisite accuracy by Sacco et al. (2008).
The pEW(Li I)s,
of
80-100 Å,
are more than five times
lower than typical
Orionis
members of the similar spectral types and
magnitudes, but still appreciable.
If the pEW(Li I)s
were correct, then the three M dwarfs should be of
the age of the Pleiades or younger (
Ma).
I instead consider the feature at the Li I
wavelength that Sacco
et al. (2008)
found was not lithium in absorption, but a collection of lines of
molecular CN (12C14N -
Grevesse 1968;
Mandell et al. 2004;
Ghezzi
et al. 2008).
The later the spectral type of a star is, the stronger the molecular
bands of
lines become.
In M1-3V stars, the CN features may be strong enough to contaminate the
lithium
region.
This CN contamination might also affect the spectra of three young
Orionis
member candidates reported by Sacco et al. (2007) that
exhibited lithium depletion at the level of the three field M dwarfs in
Table 3.
4.3 HD 294297
For completeness, I also measured the proper motion of the
late-F/early-G star
HD 294297.
Identified as a photometric non-member in the
Ori OB 1b association
subgroup in the early work of Warren & Hesser (1978), the
measurement of a
high lithium abundance in HD 294297 by Cunha et al. (1995),
,
was in contrast used as an
evidence of its youth. Furthermore, the star was one of the only two
``young solar-type members of the
Orion association'' that were observed with STIS onboard the Hubble
Space
Telescope to investigate their boron abundance (Cunha
et al. 2000).
However, Caballero (2007a),
who retrieved an abnormally high proper motion from
the Tycho-1 catalogue (but with generous errorbars), again casted
doubts on its
membership in Ori OB 1b.
Afterwards, González Hernández et al. (2008) derived a
lithium abundance
similar to that measured by Cunha et al. (1995),
,
but using a warmer effective temperature of
K (instead of
6150 K). In contrast to what is observed in the
HD 294297 spectra, stars of this
effective temperature barely show evidence of lithium destruction at
the age of
the Pleiades or even older (e.g., Soderblom et al. 1993).
González Hernández et al. (2008) found that
HD 294297 is
overluminous in the
versus J diagram with respect to
Orionis
members, where
is the luminosity per unit of
mass, normalised to this quantity in the Sun.
D'Orazi et al. (2008)
presented a FLAME spectrum of HD 294297, which may
help ascertain the nature of the star, but they did not discuss their
results.
In this work, I was able to use five astrometric
epochs from UKIDSS, CMC14,
2MASS, USNO-A2, and the Astrographic Catalogue AC2000.2 (Urban
et al. 1998).
Since the AC2000.2 epoch of observation of HD 294297 was
J1895.475, then the time interval between the first and last epochs was
longer than 110 years. The measured proper motion, of
(
,
,
) mas a-1,
is similar
to those tabulated in PPMX and UCAC3 (Zacharias et al. 2009)
, which were not available
in
2007.
The proper motion is inconsistent with membership in any young
association
subgroup in Orion.
HD 294297 is probably a late-F field dwarf at a different
heliocentric distance
than Ori OB 1b.
Table 3: Interloper M dwarfs with supposed lithium in absorption.
4.4 Caveats
Table 4: SIMBAD stars with USNO-B1 proper motions between 30 and 40 mas a-1.
In Sect. 1,
I noted the importance of a correct
de-contamination before deriving any typical parameter in studies of
star-forming regions.
For example, special care has been applied when measuring the shape of
the
initial mass function in Orionis
(see extensive de-contamination
discussion in, e.g., Caballero et al. 2007; Lodieu
et al. 2009;
and Bihain et al.
2009).
However, I warn against the incorrect use of certain
parameters that were
derived without a correct de-contamination:
Total number of stars.
Some authors, such as Sherry et al. (2004) and Walter et al. (2008), estimated that the total numbers of stars and brown dwarfs in

Frequency of discs.
According to Scholz & Eislöffel (2004), 5-7% of





4.5 Going further in terms of proper motion
In Sect. 2.1,
I noted that there were 33 USNO-B1 sources
within the survey area that have proper motions between 30 and
40 mas a-1 (and 119 between 20
and 40 mas a-1, and 450
between 10
and 40 mas a-1). With Aladin,
I searched for the SIMBAD counterparts of the 33 candidate
interlopers.
Ten of them had been discussed in the literature (Table 4), four having
been tabulated already as interlopers in the Mayrit catalogue (three of
the four have spectroscopy).
The other six interloper candidates were likely contaminants in the
works of
Sherry et al. (2004
- one interloper) and Hernández et al. (2007 - five
interlopers), which casts additional doubt on, e.g., the frequencies of
discs
derived in those studies. We clearly require a massive astrometric
follow-up of as many as possible
Orionis
member candidates that have no known features of youth.
5 Conclusions
I have used the Aladin Virtual Observatory tool to search for
proper-motion
contaminants towards the Orionis
cluster.
Of the 5421 USNO-B1/2MASS cross-matched sources in an area of radius
30 arcmin centred on the Trapezium-like star of eponymous
name, only 42 of them
had USNO-B1 proper motions larger than 40 mas a-1,
2MASS J-band
magnitudes brighter than 15.5 mag, and more than three USNO-B1
detections.
I have astrometrically followed them up using a number of
catalogues (e.g.,
USNO-A2, GSC2.2/2.3, 2MASS, DENIS, CMC14, UKIDSS) and photographic
plate
digitisations from SuperCOSMOS, and measured accurate proper motions
with
errorbars of less than 2 mas a-1.
The USNO-B1 proper motions of several targets were affected by
partially
resolved (visual) multiplicity or faintness in the visible.
Of the 42 investigated objects, 27 had previously been
considered to be
Orionis
member candidates. The other 15 objects are reported here for the first
time.
I divide the 42 objects into:
- One bona fide cluster member
of low actual proper motion (
mas a-1) with known spectroscopic features of youth (No. 36/Mayrit 1493050).
- Two cluster member candidates of low proper motion, including a brown dwarf (No. 32/Mayrit 999306 and No. 42/Mayrit 1610344).
- Two previously unknown stars of low proper motion with magnitudes and colours inconsistent with cluster membership (Nos. 04 and 38).
- 37 proper-motion interlopers, of which 21 are firstly discarded for cluster membership here. In a few cases, the proper-motion measurement is supported by spectroscopic information (e.g., lack of lithium in absorption) obtained in other works.

I discussed the curious resemblance between the
pseudo-equivalent widths of
the Li I 6707.8 Å line in
absoprtion of three early-M
proper-motion contaminants and three ``lithium-depleted young star''
candidates.
I also measured the proper motion of the star
HD 294297 with a time baseline of
more than 110 years and concluded that it is not a
young F/G star, but a
late-F field dwarf unrelated to Ori OB 1b.
Finally, I showed some examples of studies in which
de-contamination (by
proper-motion interlopers) was not correctly accomplished, and the
consequent implications of their results.
Preliminary analyses of stars with USNO-B1 proper motions between 30
and
40 mas a-1 indicate that
contamination by foreground interlopers could
severely affect the results of some widely used works.
This analysis is a new, but necessary, step to characterise in
an optimal
way the stellar and substellar populations of the Orionis
cluster,
which is one of the most accesible laboratories of star formation.
I am grateful to the anonymous referee for his/her fast and considerate report. I thank J. I. González Hernández for helpful discussion on lithium abundance. Formerly, I was an investigador Juan de la Cierva at the Universidad Complutense de Madrid; currently, I am an investigador Ramón y Cajal at the Centro de Astrobiología (CSIC-INTA). This research has made use of the SIMBAD, operated at Centre de Données astronomiques de Strasbourg, France, and the NASA's Astrophysics Data System. Financial support was provided by the Universidad Complutense de Madrid, the Comunidad Autónoma de Madrid, the Spanish Ministerio Educación y Ciencia, and the European Social Fund under grants: AyA2008-06423-C03-03, AyA2008-00695, PRICIT S-0505/ESP-0237, and CSD2006-0070.
Appendix A: Long tables
Table A.1:
USNO-B1 stars towards Orionis
with
mas a-1,
mag, and
:
coordinates, magnitudes, and proper motions.
Table A.2:
USNO-B1 stars towards Orionis
with
mas a-1,
mag, and
:
alternative names and cluster membership status.
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Footnotes
- ...2009)
- Besides,
Zapatero Osorio et al. (2008) used the small proper motion of S Ori 70
to indicate that this object is farther away than expected if it were a single
field T dwarf lying in the foreground of
Orionis.
- ...
systematics
- For example, all LNPM2 stars in the survey area except two
have very low proper motions.
The two exceptions, with tabulated LNPM2 proper motions of about
10 mas a-1, are an anonymous binary with true
and G 99-20, a high proper-motion star with true
mas a-1 (see below).
- ... faintness
- The large
uncertainties in the proper motions of targets fainter than
mag is an a posteriori justification of the limit J < 15.5 mag imposed in Sect. 2.1.
- ...
TYC 4770 924 1)
- The same can be applied to the non-Tycho-2 star HD 294276, which was recovered from the Tycho-1 catalogue.
- ... coordinates
- At least the following
Orionis binary members with
arcsec should also have very large spurious proper motion values in a 2MASS/UKIDSS cross-match: Mayrit 707162 AB, Mayrit 1564349 AB (Caballero 2006), Mayrit 1245057 AB, and Mayrit 1411131 AB (Caballero 2008c).
- ...2009)
- Cunha
et al. (1995) reported a proper motion (
,
, -1.8) mas a-1.
All Tables
Table 1: Astrometric epochs for star No. 24 (G 99-20).
Table 2: Companions in visual binary and multiple systems with incorrect USNO-B1 proper motions.
Table 3: Interloper M dwarfs with supposed lithium in absorption.
Table 4: SIMBAD stars with USNO-B1 proper motions between 30 and 40 mas a-1.
Table A.1:
USNO-B1 stars towards Orionis
with
mas a-1,
mag, and
:
coordinates, magnitudes, and proper motions.
Table A.2:
USNO-B1 stars towards Orionis
with
mas a-1,
mag, and
:
alternative names and cluster membership status.
All Figures
![]() |
Figure 1:
Inverse-colour digitised image of the red plate (RJ)
of the Digital
Sky Survey II centred on the Trapezium-like |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Inverse-colour SuperCOSMOS digitised images of the red plates (RJ)
of POSSI (J1951.9, left) and UKST (J1991.0,
right) centred on the
J2000 position of star No. 24 (G 99-20). Size is
5 |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Right ascension and declination as a function of modified Julian date for the star No. 24 (G 99-20). Dashed (red) lines indicate the linear fits. The largest errorbars correspond to the UKST Blue (GSC2.2/2.3) astrometric epoch. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Proper motion diagram of the 42 main targets.
Typical errorbars are of the same size as the small (blue) dots.
The (red) circle and small square close to the origin of coordinates
indicate |
Open with DEXTER | |
In the text |
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