A&A 445, 1081-1088 (2006)
DOI: 10.1051/0004-6361:20053901
A. Richichi1 - O. Fors2,3 - M. Merino2 - X. Otazu4 - J. Núñez2,3 - A. Prades5 - U. Thiele6 - D. Pérez-Ramírez7 - F. J. Montojo8
1 - European Southern Observatory,
Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2 -
Departament d'Astronomia i Meteorologia, Universitat de Barcelona,
Martí i Franqués 1, 08028 Barcelona, Spain
3 -
Observatori Fabra, Camí de l'Observatori s/n, 08035 Barcelona, Spain
4 -
Computer Vision Center, UAB, Bellaterra, Spain
5 -
Escola Universitaria Politècnica de Barcelona,
Universitat Politècnica de Catalunya, Barcelona, Spain
6 -
Calar Alto Observatory, Almería, Spain
7 -
Universidad de Jaén, Dpto. de Física, Campus Las
Lagunillas s/n, 23071, Jaén, Spain
8 -
Real Instituto y Observatorio de la Armada, San Fernando, Spain
Received 25 July 2005 / Accepted 12 September 2005
Abstract
We present an update of the lunar occultation program which
is routinely carried out in the near-IR at the
Calar Alto Observatory. A total of 350 events were
recorded since our last report (Fors et al. 2004, A&A, 419, 285).
In the course of eight runs we have observed, among others,
late-type giants, T-Tauri stars, and infrared sources.
Noteworthy
was a passage of the Moon close to the galactic center, which
produced a large number of events during just a few hours in July 2004.
Results include the determinations of the angular diameter
of RZ Ari, and the projected separations and
brightness ratios for
one triple and 13 binary stars,
almost all of which representing first time detections.
Projected separations range from
to
.
We provide a quantitative
analysis of the performance achieved in our observations in terms
of angular resolution and sensitivity,
which reach about
and
mag,
respectively.
We
also present a statistical discussion of our sample, and
in particular of the frequency of detection of binaries among
field stars.
Key words: astrometry - occultations - binaries: close - binaries: visual
In this paper, we provide an overview of the data we have accumulated since our last publication on the subject (Fors et al. 2004, F04 hereafter), adding 350 LO events. We present new approaches in data analysis, suited to the automated reduction of large volumes of LO, and we provide details on 11 new binaries and one triple star, 2 known binaries, and one angular diameter determinations. We also discuss the statistics of our sample, with some considerations on the detection of binaries in random observations of field stars.
Table 1: Observing runs.
Observations were carried out during eleven observing runs at the Calar Alto Observatory over a period of two years, as detailed in Table 1. On average, each run consisted of a few nights allocated in periods of crescent Moon close to full phase, in order to maximize the number of occultations of field stars and observe disappearances rather than reappearances. On two occasions (runs G and H), very short runs were allocated to follow up passages of the Moon close to the galactic center and over the Taurus star-forming regions, respectively. Note that three runs were completely devoid of results due to weather. The two earlier results from TIRGO are collectively grouped as run A, although their respective observations were collected at different dates in 2001. Details of the 350 recorded events and the characteristics of the corresponding objects can be found in Table 5, available only on-line. Here we show an excerpt in Table 2, which contains the details of the sources explicitly mentioned either for a positive result or for other comments. The format of this table is similar to the one used in F04. In Col. (3), the codes T, CB and CC are for the TIRGO telescope equipped with a fast InSb photometer, and for the Calar Alto 1.5 m and 2.2 m telescopes equipped with MAGIC cameras, respectively.
Table 2: List of selected occultation events and of the circumstances of their observation.
The availability of near-IR all-sky surveys has represented
a major step forward in the possibilities of LO investigations,
especially at medium and large-sized telescopes. Roughly speaking,
a 1.5 m telescope equipped with an InSb fast photometer can
record LO events with millisecond time sampling and signal-to-noise
ratio (SNR) above unity for sources having magnitudes (Richichi et al. 1996). An instrument based on the fast readout
of a subwindow of an array detector can add more than one
magnitude in sensitivity (F04). Moving to larger
telescopes brings gain in sensitivities which are essentially,
for moderate lunar phases and faint sources, proportional to
the area of the telescope (Richichi 1996). While these guidelines
are obviously strongly dependent on a number of instantaneous
parameters, they show that LO observations can be adequately
recorded on sources with magnitudes as faint as
.
Until recently, no comprehensive coverage of the near-IR sky
was available. The Two Micron Sky Survey (TMSS,
Neugebauer & Leighton 1969)
was incomplete in declination and only extended to
.
In the
past, LO
predictions were compiled by the present authors using a variety
of other catalogues. Even a very rich run would consist of
about 10-20 sources per night at most.
We have now implemented the 2MASS survey (Cutri et al. 2003) in our
predictions, and the number of events observable has jumped
up by very large factors. A typical night would offer in
excess of 100 sources close to maximum lunar phase. Even more
dramatic are the improved conditions for special events. For example,
on the occasions of passages of the Moon in crowded regions
near the Galactic Center
(see Sect. 3.8),
thousand of events would be easily accessible
to a medium-sized telescope over few hours.
Table 3 lists some statistics for such
events in the near future.
In this regime, the number of events effectively
observable will depend on the overheads of telescope pointing,
instrument operation and data storing.
Of course, the increase in the number of observed events is not reflected linearly in the number of results, such as the positive detection of field binaries, mainly because the majority of the events will be faint and offer limited dynamic range. We will return to this point in Sect. 4. The increase in the sheer number of observed events, on the other hand, implies a significant load of data inspection and analysis, particularly since data from IR arrays are substantially more demanding than those from photometers. This has prompted us to handle the bulk of raw data by means of automated processing. A new reduction pipeline was designed and implemented for the automated generation of preliminar lightcurve fits, which are then improved interactively. In particular, we concentrated our effort in two different areas. On one hand, we performed a comparative study of different algorithms of light curve extraction, such as aperture photometry, Gaussian profile fit, object detection based on segmentation analysis and subtraction of fixed number of faintest to brightest pixels. The latter was found to offer the best performance over the range of SNR present in our data sets, using 30 and 15 pixels for the extraction of the background and star signals, respectively. On the other hand, a new algorithm was developed to estimate automatically the lightcurves parameters (occultation time, stellar and background intensity). A particular wavelet transform of the lightcurve was chosen for this purpose, as it was capable of isolating the desired frequency signature while preserving the temporal information. The algorithm showed great robustness even in worst SNR conditions. This work will be described in detail in a separate paper.
Table 3:
Statistics of three passages of the Moon
in the Galactic Center (GC) region in 2006, as
predicted for the Calar Alto Observatory
to the limit
mag.
The data were analyzed by means of various methods, as already described in Richichi et al. (2002, 2003) and references therein. The main engine for data analysis is based on a model-dependent least squares method (LSM). Free parameters include the stellar intensity, the rate of the event, the intensity of the background and its time drift. For single stars another parameter is the angular diameter, and additionally for binary stars the angular diameter of the companion, the projected separation and the brightness ratio are included. Spurious frequencies due to pick-up of mains power and other effects may be present occasionally, and can be digitally filtered. Relatively slow, random fluctuations of the background (due to thin cirrus and lunar halo) and of the stellar intensity (due to image motion and scintillation), can be fitted and accounted for by means of Legendre polynomials as described in the above mentioned papers. Another approach is to use a model-independent method (CAL, Richichi 1989), which is particularly suited for the detection or confirmation of companions at very small separations. This method is also of great advantage in cases when the source may not be a simple circular disk, or in the presence of extended circumstellar emission.
The stars for which a positive result could be obtained are listed in
Table 4, using the same format already used
in F04.
In summary, the columns list the absolute value of the fitted linear rate of
the event V, its deviation from the predicted rate
,
the local lunar limb slope
,
the position and
contact angles, the signal-to-noise ratio (SNR). For binary detections, the
projected separation and the brightness ratio are given, while for
RZ Ari
the angular diameter
is reported, under the assumption of
a uniform stellar disc.
All angular quantities are computed from the fitted
rate of the event. Only in
2MASS 04264187 +2500314 we were not
able to reliably fit a rate, due to the low SNR.
For this source, the
predicted values
are listed in parentheses.
Note that in the tables of this paper the 2MASS prefix is
omitted.
Table 4: Summary of results.
We can only conclude that the companion is generally oriented towards
the North, at a separation that could be significantly larger
than the projected value of Table 4, up to
50 mas. Attempts to confirm the true position angle
by techniques such as speckle interferometry are possible.
From the two events we have reliable magnitude
differences both in the R and the K bands. This permits us
to infer that the secondary is bluer, by
mag,
than the primary. The primary is classified as a K5 giant
(Houk & Smith-Moore 1988), therefore we estimate that
the secondary should have
,
which would
be consistent with a late A or early F star.
The star is an irregular long-period variable, although
the amplitude is relatively small (0.6 mag,
Kukarkin et al. 1971).
In the near-IR the amplitude of variability is
not well documented, and it can be assumed to be
even smaller. An examination of the data available
from the AAVSO shows a slight trend of increasing
luminosity by about 0.5 mag over the past 30 years
in which diameter measurements are available.
Neglecting in a first approximation
significant changes of angular diameter due to
variability, we plot all available determinations
in Fig. 1, using UD values.
The conversion from LD and FD to UD has been done
by using guidelines and conversion factors
provided in the original
references. The uncertainties
in this conversion can be considered smaller than the
error bars on the diameter determinations.
It can be noted that there is a general agreement
among the various determinations.
A weighted mean yields the UD value
mas.
![]() |
Figure 1: Angular diameter determinations for RZ Ari. The filled circle is our result, while the open symbols are: square Africano et al. (1975), pentagon Beavers et al. (1981), triangles Ridgway et al. (1980), circles Dyck et al. (1998). |
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No definite trend of the characteristic size
with wavelength seems to be present, as
would have been expected in the presence
of circumstellar matter, due to scattering at
shorter wavelengths and thermal emission at longer ones.
Therefore we can conclude that circumstellar matter
is not dominant. This is independently confirmed by
mid-infrared spectra, that show a featurless continuum
around 10 m (Speck et al. 2000).
Also, there seems to be
no evidence of binarity, a possibility which had
initially been postulated on the basis of Hipparcos
results. Percy et al. (2002) have discussed
the origin of the problem with the Hipparcos data.
Also speckle interferometry investigations
by Mason et al. (1999) did not find
companions.
From our LO result, we can put an upper limit of
1:40 on the brightness ratio of a hypothetical
companion with a projected separation in the range
70 mas.
RZ Ari has been used as a building block
in several empirical
calibrations, such
as those by
Barnes (1976, 1978),
Ridgway et al. (1980),
Di Benedetto (1993).
Dyck et al. (1998) provided a revised value
of the bolometric flux, and using their own LBI diameter
derived
K.
Of course, diameter variations must exist in this star,
and therefore it
seems of secondary importance at this point
to discuss the accuracy of the various determinations and
to refine the
value.
It would be more important to follow
diameter and temperature variations with a
dedicate monitoring, a possibility which is made
available by several of the current interferometers.
We also mention that we have detected binarity in three
further
stars from Table 2, namely
SAO 109617,
SAO 110089 and
SAO 78540. These are relatively wide systems,
with separations of order
,
and therefore easily
accessible to standard observations. For this reason, and
also because LO are not very accurate for such large
separations due to possible differences in local limb slope
for the two components, we have not included these results
in Table 4. However, we consider it
possibly useful to report the brightness ratios in the
K band. The values are
,
and
,
in the above order.
It is noteworthy that all three stars have been measured
at visual wavelengths by speckle interferometry
and/or by Hipparcos.
We quote, among others,
values of
1.66 mag (G band,
Balega et al. 2004) and
1.87 mag (Hp band,
Fabricius & Makarov 2000) for
SAO 109617,
and
p values of
0.49 mag and 1.73 mag for
SAO 110089 and
SAO 78540, respectively
(Fabricius & Makarov 2000).
We note that these latter authors provide also Tycho Band V magnitude differences.
We do not speculate at this point on the combination
of all these values with our K-band determination, in view
of the diversity of spectral bandpasses used in the visual.
A number of stars from Table 5 are additionally wide binaries with separations of several arcseconds, and we do not concern ourselves with them here.
SAO 164601 is a spectroscopic binary, which was previously
observed as double by Evans et al. (1986). These authors
reported a separation close to 1 mas, although without information
on the brightness ratio. We have analyzed our trace (
)
with
both the LSM and CAL methods, without finding evidence of binarity.
In any case, due to the near-IR wavelength and the relatively
slow sampling, we are insensitive to separations of less than
about 3.5 mas on this trace.
We notice that the position angle of our event (
)
was
almost orthogonal with that of the event observed
by Evans and collaborators.
We also recorded occultations during the passage of the
Moon over two regions of special interest.
On July 28th, 2004 the Moon reached a minimum distance
of
from the Galactic Center. In this crowded,
heavily obscured region
we could record 54 events at the 2.2 m telescope in 3.4 h,
being limited by overheads in telescope pointing and data storing.
The majority (50) of the objects has no counterpart in
optical catalogues. Spectral types on the other hand are known
for about half the sample, thanks mostly to the work of
Raharto et al. (1984). With very few exceptions,
the stars are all of M spectral type.
From the photometry available in the 2MASS catalogue
(Cutri et al. 2003), it can be observed
that about half of the stars have a color J-K>1, indicating
significant reddening. This is presumably due to interstellar
dust in the direction of the Galactic Center, however in
some cases colors as red as
J-K= 3.5-5.0 are present, possibly pointing to
additional circumstellar extinction.
In January 2005, we were able to record a passage of the Moon over the Taurus star-forming region. These passages are relatively frequent, and have been used in the past especially to derive important insights on the frequency of binaries in the early stages of stellar evolution (see Simon et al. 1995, and references therein). In our case, included were the following known young stellar objects: LH 98-106, DL Tau, GN Tau, Elias 3-18, ITG 31, LkHA 332. A few IR sources without optical counterpart were also recorded. Unfortunately, the sensitivity offered by the 1.5 m telescope was not sufficient to obtain quantitative results. Details on the full sample of occulted objects can be found in Table 5, available only on-line.
Similarly to what was done in F04,
we have computed also
the limiting angular resolution associated with the
unresolved sources,
following the same approach of Richichi et al.
(1996).
This has been done for 103 stars in our sample having
,
or
about 4 times more numerous than in the sample of F04. The result, including a comparison
with this latter work, is shown
in Fig. 2.
![]() |
Figure 2:
Limiting resolution for the sources in our sample, as a function
of SNR (solid dots). Only points with
![]() |
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It can be noted that the current sample and the previous
from F04
have an almost identical
distribution of limiting resolution against SNR,
and can be fitted by the same log-log relationship.
This is reassuring, since the behaviour
be independent
of the source, and be determined by the instrumental
characteristics and in particular by the integration time.
The large spread in the relationship can be understood
in terms of large variations of SNR from one LO light curve
to another due to different situations of background
and also to the specific conditions
of signal extraction from the
discrete pixels of the detector.
Broadly speaking, the average relationship is such
that
ensures a limiting resolution of about
3 mas. In the few cases in which SNR
close to 100
could be recorded, the limiting resolution improves but
remains above, as already noted
in F04, the performance of
fast InSb photometers which can operate with faster
sampling.
A final consideration can be made about the statistics of binary detections in our sample. We have observed a total of 14 binaries (counting as such also the triple star IRC -30319), out of a total sample size of 350 stars. This points to a fraction of 4.0%, or more than two times smaller than what observed by Richichi et al. (1996) and in F04. This result seemed puzzling at first, since all the samples considered have a broad sky distribution and should have similar characteristics. It is not excluded that the targets that we observed in the direction of the Galactic Center have an actual deficit of binaries, due to the fact extinction introduced a bias towards stars that for a given apparent magnitude are more distant than in the previous samples. Therefore, hypothetical companions would have smaller angular separations for the same statistics of semi-major axis. However, only 20% of the stars in our sample were observed in the direction of the Galactic Center, and another explanation must exist for the lower binary fraction that we observe in the present work.
In fact we note that with the introduction of large, deep IR catalogues such as 2MASS in our predictions, we have effectively shifted the distribution of K magnitudes in our sample much closer to the limiting sensitivity of the technique. Therefore, we can expect that most of the LO light curves will have on average lower SNR than in the previous samples. As a result, it will become effectively more difficult to detect companions, especially those with brightness ratios larger than unity. Although we have not performed a detailed computation of this effect, its magnitude could easily explain the observed apparent deficit of binary detections. We conclude that the introduction of large catalogues, while increasing the number of predictions and correspondingly of observed LO, does not automatically produce a higher rate of results.
The results include the detection in the near-IR of
one triple and
13 binary systems.
For all but two stars, these represent first
time detections. Projected separations range
from
to
,
and brightess
ratios reach up to 1:20 in the K band.
We have also determined the angular diameter of
the M 6 star
RZ Ari, which we have discussed in
comparison with previous determinations.
Our observations have included a passage of the
Moon over a crowded region in the vicinity of
the Galactic Center (resulting in 54 events observed
in about 3 h), and a passage in the Taurus
star-forming region.
Passages of the Moon close to the Galactic center
are taking place in these years, and we have provided
some examples. These events provide a unique
opportunity to extract milliarcsecond resolution
information on a large number of objects in obscured,
crowded and relatively unstudied regions, and can
be adequately observed with 2-4 m -class telescopes.
We have discussed the performance achieved in our
observations in terms of limiting magnitude and
angular resolution. We have shown that at 1-2 m class
telescopes equipped with a rather traditional
array detector it is possible
to achieve
on sources as faint
as
mag. The rate of binary detection
in random observations of field stars that emerges
from the present work is
4%, considerably
lower than established earlier by similar
studies
(Richichi et al. 1996;
Fors et al. 2004).
We attribute this effect largely to the fact that the use
of catalogues such as 2MASS has increased dramatically
the number of occultation observable per night, but this
increase is realized mostly at the faint magnitude
end, where the dynamic range available is much
smaller than for brighter stars.
Acknowledgements
We thank the Observatorio Astronómico Nacional (OAN) and Centro Astronómico Hispano-Aleman (CAHA) for the facilities and support made available at Calar Alto, and in particular Santos Pedraz (CAHA) for his invaluable help on many occasions. 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. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. This research has made use of the Simbad database, operated at CDS, Strasbourg (France), and of the Washington Double Star Catalog maintained at the U.S. Naval Observatory. The paper has been improved by several detailed references provided by the referee, Dr. B. Mason.
Table 5: Complete list of occultation events and of the circumstances of their observation.