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Irregular variables of type Lb. New JHKL'M-photometry for 160 stars
January
Irregular variables of type Lb. New J H K L0M -photometry for 160 stars?
F. Kerschbaum 2
C. Lazaro 0 1
P. Habison 2
0 Instituto de Astrof sica de Canarias , 38200-La Laguna, Tenerife , Spain
1 Dpto. de Astrof sica, Fac. de F sica, Universidad de La Laguna , 38200-La Laguna, Tenerife , Spain
2 Institut fu ̈r Astronomie der Universit ̈at Wien , Tu ̈rkenschanzstra e 17, A-1180 Wien , Austria
| This paper presents new near infrared observations of 160 Irregular variables of type Lb in the J HKL0M lter bands. These measurements are supplemented by data for additional 56 stars taken from the literature. In total 220 datasets are available because of some multiple observations. From our sample, 216 stars have near infrared (NIR) photometry now. Our sample of visually bright Lb-variables displays very similar infrared properties when compared with SRa- and SRb-variables. Derived from NIR-two colour diagrams the oxygen-rich Lbs seem to have intermediate atmospheric conditions between Miras and normal giants. There may be a sligthly larger \contamination" with non AGB-giants than in the case of the semiregulars. Using only our IR-colours the S- and the Carbon-stars among the Lbs again are undistinguishable from SR-variables of the same chemistry.
1. Introduction
For a long time Mira variables, OH/IR- and
Carbonstars were the most frequently studied Asymptotic Giant
Branch (AGB) objects. The Semiregular (types SRa and
SRb) and the cool Irregular variables (Lb) { quite
numerous groups of objects { have been almost neglected
although their role within the evolution on the AGB and
their overall properties are far from being understood. The
can provide important constraints for theoretical models
due to their di erent pulsational behaviour compared to
the more frequently studied Mira variables.
Fortunately, this situation has changed during the
last years { at least in the case of the Semiregulars. In
Kerschbaum & Hron (1992
, 1994, 1996, SR-Papers I, II,
III respectively) and
Kerschbaum (1995
, SR-Paper IIb)
stellar properties of Semiregular variables (SRVs) derived
from GCVS4
(Kholopov et al. 1985{88)
,
IRAS-PSC (1988
)
and
IRAS-LRS (1986
) as well as new near infrared
photometry were used to divide this inhomogeneous group of
objects into physically distinct classes and to probe their
evolutionary status. A study by
Jura & Kleinmann (1992)
came to similar conclusions concerning the galactic
distribution of these stars.
Complementing our work on SRVs, cool Irregular
variables should be analysed in an analogous way. From what
is known about the luminosities of Irregular variables of
type Lb and since this group contains also Carbon stars,
a signi cant number of Lbs should be on the
thermallypulsing AGB
(see the review by Querci 1986)
. However,
Little et al. (1987)
found no Technetium in the O-rich Lbs
of their sample whose presence would be an evidence for
a recent thermal pulse.
Peters (1991)
used data from the IRAS-mission for an
analysis of the space distribution and the mass loss of the
main three AGB-variables the Miras, the Semiregulars and
the Irregulars. The main outcome concerning the latter is
that Irregulars seem to have mass loss rates comparable to
those of Semiregulars but smaller than those of Miras. He
arrived at similar scale heights for all three groups except
for the long period Miras which turned out to be more
concentrated to the disk.
In a study mainly devoted to Semiregulars,
Jura &
Kleinmann (1992)
derived a galactic distribution of the
Irregular variables comparable to that of \thin disk" Miras.
Unfortunately, their sample was a mixture of Irregulars
with a signi cant number of Semiregulars and Miras with
unknown periods. Moreover, it was limited to objects with
jbj 30 having GCVS and IRAS-data. All these
restrictions did not allow a more detailed analysis. Nevertheless,
these examples of recent publications in that eld demon- authors during three dedicated runs in 1994 and 1995 and
strate the need for more work. an additional one (November `94) again in collaboration
Since our approach needs data from the visual to the with C. Loup. These datasets are labeled `I' in Table 3.
far infrared the rst step was to close the gap in the stars
energy distributions between the visual GCVS4 data and
the 12−100 m infrared range of the IRAS-PSC. We ob- 3. Oxygen-rich Lb-variables
tained new JHKL0M -photometry and supplemented it
by `photometrically compatible' data from the literature
for a representative subsample of the Lb variables in the
GCVS4. In order to derive bolometric magnitudes for
volume limited samples of Lbs (compare SR-Paper I) { which
we plan for a forthcoming paper { the objects presented in
the paper will also act as prototypes providing us with the
bolometric corrections needed for the stars lacking
NIRdata.
2. Observations
A sample of optically bright GCVS4 variables of type Lb
was selected by means of the availability of good
photometry in the IRAS- (...truncated)