A&A 380, 526-532 (2001)
DOI: 10.1051/0004-6361:20011451
A. Richichi1 - G. Calamai2
1 -
European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching bei München,
Germany
2 -
Osservatorio Astrofisico di Arcetri, L.go Enrico Fermi 5, 50125 Firenze, Italy
Received 1 August 2001 / Accepted 8 October 2001
Abstract
In the framework of an ongoing series of high angular resolution
observations by lunar occultations in the near infrared, we present
accurate angular measurements
for ten late-type giant stars, seven of which were measured
for the first time.
The sample includes one K and seven
M stars. It also includes two S stars, representing to
our knowledge the first direct determination of the angular
diameter for this class of stars.
The measured angular
diameters range from 2.50 to 4.82 milliarcsec, with
an average accuracy <5%. An exception is represented
by the case of a star with a faint companion, where we have been
able to investigate in detail the bias on the diameter determination,
and hence on the effective temperature, which would be caused
if the companion had not been included in the analysis. We
suggest that faint undetected companions could explain at
least some of the hitherto known cases of stars with
effective temperatures remarkably lower than the standard
calibration.
For seven of the stars in the sample, we have used
our own photometry in conjunction with values from the
literature, to compute the bolometric fluxes and the
effective temperatures, which we discuss in the context of
existing calibrations.
With the achieved accuracy level in the angular
diameter, such measurements are also
valuable as calibrators or science verification targets for
modern large ground-based interferometers.
Key words: stars: fundamental parameters - stars: late-type
This paper is the continuation of a series, dedicated to angular diameter measurements of late-type stars obtained by the method of lunar occultations (LO). Previous papers included or directly related to the series can be found in Richichi et al. (1998). The main aim is to obtain reliable angular diameters in a highly homogenous way by using a few instruments with well-understood properties, and by using one single data analysis method which includes corrections for several potential biases and a consistent definition of the formal errors.
The results of this observational program have been used mainly to derive, in conjunction with the bolometric fluxes of the sources inferred often by means of our own photometry, the effective temperatures of K and M giants and carbon stars. A temperature scale, derived in a homogenous way using our LO measurements only, was presented by Richichi et al. (1999). Additional measurements of other properties, such as limb-darkening, circumstellar shells and photospheric pulsation, have also been obtained for some stars and have been detailed in other papers of this series.
Another aim of this program is to identify objects suitable for calibration of modern long-baseline interferometers, as well as for follow-up investigations. Large facilities such as CHARA, Keck and VLTI are active or about to begin operations, and they will explore a range of sensitivities and angular resolutions, the combination of which has been accessible until now only to LO observations. It is therefore important to establish a database of sources already measured by an independent technique such as LO. Some of the sources in this series of papers, moreover, should be investigated more fully because of variability or non symmetrical geometry, and long-baseline interferometers are in an ideal position for this.
With the present paper, we add accurate angular measurements for ten late-type giant stars, with spectral types K, M and S. We also compute, when possible on the basis of available photometry, their bolometric fluxes and effective temperatures. The method of observation and the data analysis are briefly summarized in Sect. 2. More details can be found in the previous papers of this series. In Sect. 3 we present and discuss the results individually for each star. A discussion in the context of existing effective temperature calibrations, as well as with respect to large ground-based interferometers, is given in Sect. 4.
Table 1 summarizes the details of the observations, following the format of previous papers (see for instance Richichi et al. 1998). In Cols. (1) and (2) we list the source name and the date of the event. Column (3) lists the symbol to identify the telescope used, which for the stars presented here was always the 1.5 m TIRGO telescope. In Cols. (4) through (7) we list the predicted position angle of occultation, the aperture of the photometer, the sampling time of the light curve and the integration time of each data point.
All occultations were observed with the TIRGO facility fast photometer, in a standard broad-band K filter. Details can be found in Richichi et al. (1997), and references therein. The same photometer was also used to obtain near-IR photometry of some of the sources in our sample, as described in Sect. 3.
The analysis of the LO data was carried out, as in the other papers of this series, by a program based on the least-squares method. The program includes correction of scintillation, of pick-up frequencies, and of the bias due to the finite time response of the instruments when required. These features, as well as the details of the error estimation, have been described in Richichi et al. (1992).
(1) | (2) | (3) | (4) | (5) | (6) | (7) |
Source | Date | Tel. | PA | D | ![]() |
![]() |
UT |
![]() |
![]() |
ms | ms | ||
YZ Ari | 07-01-98 | T | 110 | 21 | 2.41 | 2.00 |
EI Tau | 10-01-98 | T | 78 | 21 | 2.68 | 2.26 |
W Tau | 05-11-98 | T | 244 | 28 | 2.41 | 2.00 |
SAO 93950 | 05-11-98 | T | 306 | 28 | 2.41 | 2.00 |
IRC +20083 | 05-11-98 | T | 240 | 28 | 2.41 | 2.00 |
DY Tau | 07-11-98 | T | 287 | 28 | 1.42 | 1.00 |
IRC +20156 | 19-01-00 | T | 51 | 21 | 4.42 | 4.00 |
IRC +20128 | 13-03-00 | T | 60 | 21 | 2.41 | 2.00 |
VV Cnc | 15-03-00 | T | 124 | 14 | 2.41 | 2.00 |
IRC -20422 | 10-08-00 | T | 73 | 21 | 4.42 | 4.00 |
Table 2 lists cross-identifications of the observed sources in various catalogues, as well as some photometric and spectroscopic data.
Source | TMSS | IRAS | SAO | HD | Other | V | K | Sp. | Notes |
YZ Ari | 02547+1106 | RAFGL 5087 | ![]() |
3.5 | M8 | Mira, OH/IR | |||
EI Tau | 05440+1753 | S1 116 | 13.0 | 3.9 | Svar | binary | |||
W Tau | 04250+1555 | 28236 | BD+15 628a | 9.1 | 1.1 | M6 | |||
SAO 93950 | +20079 | 04255+1614 | 93950 | 28292 | 75 Tau | 5.0 | 2.2 | K2 | binary, biased value |
IRC +20083 | +20083 | 04272+1603 | 93969 | 28484 | BD+15 635 | 7.9 | 2.0 | M3 | |
DY Tau | +20119 | 05390+1831 | 94779 | 246450 | S1 112 | 9.6 | 2.6 | M.../S | |
IRC +20156 | +20156 | 78609 | 47548 | BD+20 1521 | 8.0 | 2.3 | M0/M... | conflicting sp. types | |
IRC +20128 | +20128 | 05539+2016 | NSV 2736 | 2.1 | M6/M7 | candidate OH/IR | |||
VV Cnc | +20197 | 08083+1917 | 97631 | BD+19 1947 | 9.6 | 1.4 | M5 | ||
IRC -20422 | -20422 | 2.7 | M4 | no opt. counterpart |
Our angular diameter results are listed
in Table 3. They have been obtained
under the hypothesis of a uniform disk.
A thorough discussion of the center-to-limb brightness
variations (CLV) in non-Mira stars has been given
in Hofmann & Scholz (1998), from which it can
be inferred that at near-IR wavelengths the effects of
limb-darkening are relatively small for the sources
in our list. Therefore, we prefer
to present here the uniform disk values only, which are
clearly defined and can be converted as needed to diameters
under different limb-darkening assumptions.
For example, Richichi et al. (1999)
have used a grid of numerical CLV models to analyze a set of LO
data, and have derived a conversion factor between
uniform disk and limb-darkened disk, for different assumptions.
The conversion factors that would apply to the stars and
wavelengths ranges considered in this paper are typically
1-3
.
In this section, we present the results separately for each individual star, in connection with earlier findings and available literature. We do not present LO lighcurves and model fits for all stars, and rather we give in Fig. 1 one example for illustration. As a measure of the quality of each data set, we list in Table 3 the signal-to-noise ratio (SNR), computed as the ratio of the stellar intensity outside the occultation to the average standard deviation of the fit residuals. Additionally, we compute and list in Table 3 also the bolometric flux, when possible. Since for many of the stars in our sample little or no photometric data were available, a number of photometric measurements were obtained, also with the TIRGO telescope, and are reported in the individual subsections. Even after this effort, the photometric coverage remains insufficient, resulting in some cases in a large uncertainty on the amount of extinction or in the absence of a bolometric flux estimate. For this reason, we do not attempt in this paper to derive explicitly effective temperatures of the stars in our sample.
The LO light curve for this source shows a well-resolved angular diameter which represents a first time measurement for this star.
YZ Ari is seen as a very faint object in the PSS plates, although
the Combined General Catalogue of Variable Stars (Kholopov et al.
1988) quotes a maximum magnitude of 5. In the infrared however,
this source is quite prominent, being listed among the brightest IRAS
sources above the galactic plane (Hacking et al. 1985).
Near-IR photometry is available by Fouque et al. (1992),
as well as by Whitelock et al. (1994) who covered a wide
range of epochs. Photometry was also obtained at the TIRGO telescope
on the night after the occultation event, with magnitudes of
,
,
,
in the J, H, K, L bands respectively.
The analysis of all available near-IR data shows a semi-regular
variability, with a main period of
450
close
to the value derived
by Whitelock et al. (1994).
The source is tentatively classified as a Mira variable, and a
spectral type of M8 is reported by these latter authors, although
not confirmed by Kwok et al. (1997).
YZ Ari also shows a marked IR excess especially at 25 and
60 m, which was recognized as characteristic of a OH/IR star
by Eder et al. (1988), who detected double-lined
maser emission at 1612 MHz. Searches for SiO and OH maser
emission at 22 GHz gave negative results both with the Haystack
and the Arecibo radio telescopes (Benson & Little-Marenin
1996; Engels & Lewis 1996).
We have attempted to derive an effective temperature for this star,
under the assumption that we have indeed detected and resolved
the photosphere. For this, we have estimated the bolometric flux
at the date of the occultation using the timely TIRGO photometry,
and the IRAS fluxes which we assume to be non-variable (for
comparison, K-band variations have an amplitude of about 1 mag).
A fit to the photometric points was obtained by a
simple two black-body model; the resulting bolometric flux was
,
with the
infrared excess accounting for about 22%
of the total flux.
Note that our LO data at 2.2
m show no
evidence for extended emission on
around the
star, with a SNR of 40 (per data point). Investigation over a
wider angular range was not possible due to slow background
fluctuations.
Clearly, significant temperature changes can be expected with the variability phase and follow-up studies combining adequate angular resolution with frequent photometric monitoring would be desirable.
We obtained the first angular diameter measurement
for this S star. Together with that
of DY Tau, this is to our knowledge the first one
for a star of this spectral class.
TIRGO photometry on the same night as the LO event
showed
and
,
in the J and K bands respectively.
Unfortunately the available photometry is not sufficient to
compute a reliable bolometric flux and derive an
effective temperature for this source.
It is to be noted that, in addition to the angular diameter measurement, our LO light curve also showed the presence of a nearby companion, as described in Richichi et al. (2001).
The LO light curve for this source yielded a quite accurate angular diameter. Previously, three measurements for W Tau were obtained by means of LO by Ridgway et al. (1980, 1982). The resulting angular diameters, in the H and K near-IR bands, were between 3.81 and 4.38 mas, roughly consistent within the quoted errors among themselves, as well as with our determination. We can thus conclude that the angular diameter of this star is relatively constant.
On the nights of November 5-6, 1998, we also obtained near-IR
photometry from TIRGO, with mean values
,
,
,
in the J, H, K, L bands respectively.
Kerschbaum & Hron (1994) quote very similar
values, confirming the very limited variability, if any,
of this source in the near-IR.
We have computed the bolometric flux for this star by means of
a two components model, although we note
that the IR excess accounts for
1% of the total flux.
In this context, we also note that no water maser emission was detected
in a search by Lewis (1997). The result is
.
Kwok et al. (1997) assigned
a spectral type of M 6 to W Tau.
We have recorded two occultations of this source, and an initial report regarding the detection of a companion has been given in Richichi et al. (1998), where also a summary of previous LO observations was given. For this paper, we have analyzed in detail the first of our two LO traces to derive the angular diameter.
We have found that, in spite of its small intensity
relative to that of the primary, the presence of the companion
is indeed important in the outcome of the data analysis.
Contrary to what we had stated in
Richichi et al. (1998), the companion
is indeed detected also in this LO light curve, in spite
of some slow scintillation-induced fluctuation of the signal
(Richichi et al. 2001).
Indeed, the result for the angular diameter would be
seriously biased without taking the companion into account,
resulting in
mas.
This is probably the explanation of previous results,
such as that by
Ridgway et al. (1982), who had found a similarly
large value for the angular diameter, which in turn resulted
in a peculiarly low effective temperature for this star.
This had been remarked also by
Dyck et al. (1996), who used the LO result for
this star for their effective temperature calibration.
When the companion is included in the fit, the resulting
angular diameter for the primary is significantly smaller.
However, the actual value of the diameter depends strongly
on the brightness ratio. Just by changing
this latter from
to 3.4,
the resulting angular diameter of the primary
changes from 2.50 to 1.70 mas,
respectively.
We note that the result mentioned in
Richichi et al. (1998) concluded that
.
Obviously,
the conclusions on
this star must be refined, however we can already explain
the discrepant conclusions presented in previous publications.
Further observations
with good sensitivity are needed to put stronger constraints
on this system, and hence
obtain a bias-free angular
diameter. For the purpose of this paper, we adopt
the value
mas for the angular diameter, where the large error
is chosen to account for the scatter in the results obtained
under various assumptions.
On the same night of the LO event, we also obtained near-IR
photometry from TIRGO, with magnitudes
,
,
,
in the J, H, K, L bands respectively.
Since this star is not presumed to be variable, we have
relied on the
bolometric flux value computed by
Dyck et al. (1996).
Our photometry, obtained from TIRGO on the same night of the LO event,
showed magnitudes of
,
,
,
in the J, H, K, L bands respectively.
An accurate computation of the bolometric flux should take
into account the visual extinction, which is significant
for this source. Our computed value of
,
was
based on an estimate of
mag,
which however cannot be considered more accurate than
0.5 mag due to the insufficient photometric
coverage.
Our LO has allowed us to measure for the first time the angular diameter of DY Tau with high precision.
This is one of the brightest S stars in the sky at 2 m
(Wing & Yorka 1977), and to our knowledge
together with EI Tau the first one for which the
angular diameter could be measured directly.
Unfortunately the available photometric data are not
a good match to the precision obtained on the angular diameter.
Only Kerschbaum et al. (1996) measured this source.
The K magnitude is very close to the IRC determination.
We combined these data with the IRAS fluxes, to derive
a bolometric flux. However this is not sufficient
to impose strong constraints on the visual extinction.
A rough estimate indicates
mag. With this assumption,
the resulting flux would be
,
but
additional photometric data are required before
making use of this result.
The angular diameter of
mas
derived for this star is considerably
larger than expected for a source of this brightness with
the listed spectral type (M0 in the SAO catalogue).
We have carried out near-IR photometry for this star on
the same night of the LO event, obtaining
3.81,
2.65,
2.34,
2.07 in the J, H, K and L bands respectively
(formal errors were 0.02 mag for all filters).
After combining this with the visual and infrared fluxes
available from Tycho and IRAS, we obtain
a bolometric flux of
,
with no
indication of IR excess.
To verify this, we have also fitted the spectral
energy distribution by a single black body model, to derive
an indirect but independent estimate of the angular
diameter. The result was
3 mas,
smaller than the direct LO measurement but still in excess of
empirical estimates based on the spectral type and magnitude.
The problem is that the angular diameters, whether derived by LO or by a fit to the spectral energy distribution, lead to effective temperatures of 2200 to 2800 K, considerably lower than expected for a normal early M giant. We have analyzed the LO light curve to investigate whether a second component could be present, with negative result. A previous LO observation was recorded, without any remarks, by Eitter & Beavers (1979) in the B and V filters. The problem of the temperature of this object remains open and should be investigated by further observations. We note however that the Simbad database lists a type M..., which is an indication of a subtype much cooler than M0.
![]() |
Figure 1: The occultation trace of IRC +20128 (dots), and our model fit (solid line). The lower panel shows the fit residuals, on an enlarged scale. |
Open with DEXTER |
The LO light curve of IRC +20128 shows a well resolved angular diameter, which represents the first determination for this star.
IRC +20128 is classified as M6/M7
(Kwok et al. 1997), on the basis of its IRAS LRS
spectrum. This source is associated with DO 11744, a
star with magnitude 11.0 in a filter intermediate between
V and R. Photometry obtained at TIRGO on March 15, 2000, i.e. less
than two days after the LO event, yielded magnitudes of
,
,
in the J, H and K bands respectively.
The very red colors of IRC +20128 make it a candidate OH/IR star, but surveys for OH and SiO maser emission yielded negative detections (Chengalur et al. 1993; Jiang et al. 1996). Lewis (1992) has suggested that OH/IR colors can be mimicked by systems in which a red giant star, undergoing mass loss, is surrounded by a degenerate companion. This latter would accrete the mass loss from the red giant, giving rise to an intense UV flux that would dissociate the molecules which would give rise to the maser emission. Since our LO observation does not have the sensitivity necessary to detect such a companion, no conclusions can be made in this respect.
The spectral energy distribution shows some IR excess, which however
contributes to the bolometric flux at the level of
2% only.
A fit by a two black-body model yielded
.
We have obtained a first-time accurate angular diameter for this source. VV Cnc is classified as a pulsating variable star. The amplitude of the photometric variations is about 1.0 mag in photographic plates (Kukarkin 1971). Unfortunately we could not obtain photometry of this source from TIRGO.
Some discrepancy
exists in the literature about the spectral classification, which
ranges from
M3 (Jacoby et al. 1984; Schmidt-Kaler & Oestreicher 1998)
to M5 (Hansen & Blanco 1973; Kwok et al. 1997).
The difference could be indicative more of
intrinsic difficulties to reconcile the conclusions of
spectral classifications methods based on different criteria
and wavelength ranges, than of actual
variations in the star itself. However, we note that on the
basis of current calibrations
(see for instance Richichi et al. 1999),
the difference between M3III and M5III would amount to 200 K
in the effective temperature, or
12% in the angular diameter.
This should be compared with the 2% formal accuracy of our result,
which could then be used, in conjunction with a bolometric flux
which unfortunately cannot be computed at the present time for
lack of published photometry, to understand the spectral
characteristics of this star.
We obtained a first-time angular diameter for this source, which has no bright optical counterpart. A spectral type of M4 was assigned by Hansen & Blanco (1973). The source is clearly heavily obscured, however its location on the galactic plane in the direction of the galactic center, as well as its relatively early spectral type, indicate that the source of extinction is probably galactic dust. The lack of any photometry, apart from the IRC determination, unfortunately hampers at the moment any estimate of the bolometric flux and effective temperature for this source.
Source |
![]() |
SNR | Bolom. flux |
(mas) |
![]() |
||
YZ Ari |
![]() |
38.8 | ![]() |
EI Tau |
![]() |
47.2 | - |
W Tau |
![]() |
108.2 |
![]() |
SAO 93950 |
![]() |
95.0 |
![]() |
IRC +20083 |
![]() |
82.0 |
![]() |
DY Tau |
![]() |
109.4 |
![]() |
IRC +20156 |
![]() |
77.0 |
![]() |
IRC +20128 |
![]() |
143.4 |
![]() |
VV Cnc |
![]() |
78.2 | - |
IRC -20422 |
![]() |
70.0 | - |
The classical goal of angular diameter measurements is to
establish a calibration of the effective temperature with
spectral types.
While such a calibration has been reasonably
well established for giant stars hotter than 4500 K,
corresponding to types earlier than K1-K2III, the situation
for late K stars and especially for M giants has been
less clear.
Nevertheless, also in this area
significant improvements have been made recently,
especially thanks to the contributions of observational
techniques such as long-baseline interferometry (LBI,
see for instance Perrin et al. 1998) and
LO (Richichi et al. 1999).
At present, the calibration can be considered relatively
well established for temperatures hotter than
3300 K,
or types about M6 and earlier (in the sense that calibrations
from different authors show agreement within errors).
Even so, interesting special cases remain to be understood
(Dyck et al. 1998; Richichi et al. 1999).
The situation is much more complex for stars cooler than M6. In this range, stellar pulsations, mass loss, changes in spectral type and in general Mira-like phenomena are the norm. In addition to the spectral type, further distinctions must be made depending on the type of variability and the chemical composition (Miras, carbon stars, long-period variables, and so forth). Determinations of the effective temperature must necessarily be made as a function of time, and even the mere definition of an angular diameter can be problematic due to limb-darkening and optical depth effects. Furthermore, the presence of circumstellar emission very close to the stellar photosphere complicates the interpretation of high angular resolution observations.
In this area of research, a clear bifurcation takes place between what is investigated by LBI and by LO. In the case of LBI, which is currently limited to rather bright near-IR magnitudes, the targets of choice are usually well-studied stars, for which a wealth of information is available on the spectral classification, photometry, and variability. In the case of LO, which are more sensitive but are limited to those objects which happen to be occulted by the Moon, the targets tend to be more anonymous objects, often very poorly studied, for which it is necessary to obtain additional information before a meaningful interpretation can be attempted.
Some of the stars in our sample reflect this situation very well. For three of them, namely EI Tau, VV Cnc and IRC -20422, the available photometry is not sufficient to compute a bolometric flux, and our result is confined to the measurement of their angular diameter.
In the case of W Tau, IRC +20083, and DY Tau, we have been able to compute a bolometric flux and an effective temperature which can be considered as relatively reliable. These three stars are relatively well fitted by the current effective temperature calibrations, although especially for the last two there is space for considerable improvement of the available photometry and of the estimate of extinction.
The cases of IRC +20156 and IRC +20128 are more problematic, or more interesting depending on the point of view. Both seem to exhibit a much cooler effective temperature than expected. However, in both cases some peculiarities have been noted: IRC +20156 is a star with very conflicting spectral type determinations, and IRC +20128 has colors that mimic an OH/IR star although no maser emission could be detected.
YZ Ari is the coolest star in our sample. Both its angular diameter and its bolometric flux are sufficiently reliable, and they indicate a very low effective temperature of 1700 K. This finds some justification in the very extreme AGB character of this star, which has marked IR excess and OH/IR characteristics. Further monitoring by other techniques, such as sensitive IR interferometry, would be highly desirable.
The case of SAO 93950, an early K giant for which the effective temperature calibration has always been relatively undisputed, was in fact a puzzle since previous measurements assigned a temperature to this star which was strikingly lower than expected. We have demonstrated that the presence of a faint companion can explain this discrepancy, and have obtained an effective temperature which is much closer to the standard calibration, although further measurements of the companion are required to reduce the uncertainty on the diameter of the primary.
The solution to this apparently discrepant case
could also be the explanation of a few similar cases of
stars which were found to have remarkably low effective
temperatures
(Dyck et al. 1998; Richichi et al. 1999).
We suggest that in at least some of these cases the
determination of the angular diameter could have been
biased by an undetected faint and close-by companion.
Such cases should be investigated in detail by LBI observations
with high sensitivity and long baselines. In the case of
SAO 93950, where we could confirm the companion thanks
to an independent LO observation, the difference
between the two components is
,
and yet the
bias on the effective temperature determination is
of several hundreds degrees.
We note that apart from this latter case, the relative errors
on the angular diameters in our sample are on average <5%.
This makes them, as well as many other measurements previously
reported in this series, ideal candidates for calibration and
verification of large LBI facilities, where the emphasis will
be on relatively faint and well-known stars. To make an example,
we consider an interferometer with a baseline of 100 m working
at 2.2 m: these characteristics match well what is
expected for example in the case of the ESO VLTI.
Considering that in normal operation such an interferometer
will study objects with
squared visibilities between 10% and 80%, the range
of angular diameters of interest is 1.3 to 3.9 mas.
At the smaller end of this range, an accuracy of 5% in the
angular diameter translates to 2% in the squared visibility.
Although some of the
modern large interferometers have more ambitious goals
than this, they will need a network
of calibrators with small and accurate angular diameters,
and results such as those presented in this paper can
be a valuable first step in this direction.
We conclude that the measurements that we have presented add important novel data to the calibration of the effective temperature of late M giants. We note that in many cases the actual error on the derived effective temperature is dominated, especially for relatively poorly studied objects, by uncertainties in the bolometric fluxes and in the extinction. Extensive photometric monitoring is required to establish better estimates of these quantities, and to determine variability. This task is well suited to small telescopes equipped with near-IR instruments.
At the same time, we have shown that the stars in our sample and in the previous papers of this series have the potential to be used by large modern interferometric facilities for calibration and verification.
Acknowledgements
This research has made use of the Simbad database, operated at the Centre de Données Astronomiques de Strasbourg (CDS), and of NASA's Astrophysics Data System Bibliographic Services (ADS).