We carried out a search for the optical counterparts of the selected sources using POSS, ESO and SERC Sky Survey prints and adopting a procedure similar to that outlined by Ghosh et al. (1984). The optical candidate of the IRAS source is the one inside the IRAS error ellipse. The HST Guide Star Catalog (hereafter GSC; Lasker et al. 1990; Jenkner et al. 1990) was also searched to find the optical counterpart for the programme sources. The GSC contains stars in the magnitude range 6 to 15. We were able to find the blue and/or V magnitudes for many of these sources from GSC. In Table 1 we list the Serial Number of the source, its IRAS name, IRAS position, optical position of its counterpart determined from our search, the blue and or V magnitude from GSC (where the IRAS source had a counterpart in GSC), and the sky plate from which the GSC magnitude was obtained.
Table 1: IRAS and Optical position of programme sources
We present in Table 2 the far-infrared data on the programme
sources taken from IRAS PSC (1988). The table contains for each source
in columnwise sequential order, the serial number of the source, its
IRAS name, its flux densities in Janskys in the four IRAS bands at
12, 25, 60 and 
(when the quality factor is 2 or
higher), the
Variability Index (Var), and the Low Resolution Spectrometer (LRS)
spectral classification. The Variability Index measures the probability that
the source observed by IRAS is variable as determined by multiple 12
and 
measurements. It is seen from the data in Table 2 about
half of the sources have flux densities monotonically decreasing with
wavelength, while the remaining sources have their flux densities
peaking at 
, except in the one case where it
peaks at 
or beyond. The 
flux density of the former sources is
generally higher than 10 Jy, whereas the flux density of some of the
latter sources is less than 10 Jy. Generally the sources in the
former group are on an average brighter than sources in the latter
group at 
by several times.
Table 2: IRAS PSC data on the programme sources
Low Resolution Spectra (LRS) are available for 15 of the
programme sources (Joint IRAS Science Working group 1986).
Of the
seventeen sources in the group
thirteen have LRS spectra. Only three of the sources in the group
have LRS spectra. The IRAS data on the LRS spectral classification of
these sources is presented in Table 2.
The photometric observations of `unidentified' IRAS sources
were carried out using the CCD camera attached to the 1.02-m
telescope of the Vainu Bappu Observatory (VBO), Kavalur on several
nights. The CCD camera for these observations uses a Thomson CSF
TH7882 CCD chip (
), which has a special coating
for providing enhanced ultraviolet sensitivity. It is mounted in a
liquid-nitrogen cooled dewar.
The CCD in its imaging mode at the
f/13 Cassegrain focus of the 1.02-m telescope of VBO (at a scale of
) covers a total field of 
. We
used standard B, V, R and I filters to carry out the photometric
observations.
The photometric observations reported here were carried out on
several nights during the period January 1992 to March 1995. The
extinction coefficients were determined each night by observing
photometric standard stars. On those nights when enough measurements
were not available to obtain the extinction coefficients the mean
extinction coefficients valid for VBO, Kavalur were used. The
photometric data were reduced using the tasks `ccdred' routines
of the IRAF package. The ``DAOPHOT'' package was used to determine the
magnitude of the programme sources. In one or two cases where the
images of the optical candidates were not well isolated, we used
``DAOPHOT'' for images in crowded fields to extract their
magnitudes. These were then corrected for extinction due to air mass
(at the observed altitude) to obtain the instrumental magnitudes and
then converted to standard magnitudes (Johnson system) using
the transformation coefficients for the filter system in use. The
probable error in the magnitudes listed in Table 3 is 
, which
is for the full sample.
In Table 3, we list in sequential order, the serial number of the source, its
IRAS name, B, V, R and I magnitudes from the present
observations and the
date of observation. In Fig. 1 (click here) we present a plot of V-I versus B-V
for the sources
for which we have BVRI magnitudes from our measurements.
Table 3: BVRI magnitudes of the Programme sources
Figure 1: [V-I] versus [B-V] colours for the sources for which
we have B, V and I magnitudes from our photometric measurements. The
numbers beside the points identify the sources of our study. The
points 
refer to sources with 
and points 
refer to sources with
.
The thick line, dot-dashed line and the dashed line show the dependence
of [V-I] versus [B-V] for supergiants, giants and
Main-sequence stars, respectively.
The tick marks on these curves indicate the location of stars of
different spectral type and luminosity classes. The
reddening vector for a star of spectral type and luminosity M5V
is also indicated
Figure 2: Diagram showing the regions in the colour-colour plot of [25-60]
versus [12-25] that separate different types of stars with circumstellar
envelopes of dust and gas (adopted from van der
Veen & Habing 1990; VH).
The colours [25-60] and [12-25] are as defined by VH. The
evolutionary track is indicated by the curve
(thick line) 
which represents the observed colours very well for
-1.1<([12-25])<1.3.
The thin line shows the loci of black bodies with temperatures ranging from
10000 K to 150 K with specific values shown at the tick marks
on the line. The solid points indicate
the position of the programme objects to indicate their state of
evolution. The numbers beside the points identify the different
sources as listed below.
The programme sources are seen to be distributed over almost the entire
region of the VH diagram. However, except for the six stars in
region I of the
VH diagram, the rest appear evolved to different degrees
van der Veen & Habing (1988, hereafter VH) carried out a study of the
IRAS sources with CSE from an evolutionary point of view. They classfied
the IRAS
sources into ten different regions based on
their location in the [25-60] versus [12-25] colour-colour diagram. In
their notation [25-60] and [12-25] refer to
,
and 
respectively, where the
's are the IRAS flux densities at 60, 25, and 12
m uncorrected for colour dependence. Stars with oxygen rich
envelopes form a sequence in this colour-colour diagram and has been
interpreted as an evolutionary sequence of mass loss rate (Olnon et
al. 1984; Bedijn 1987;
van der Veen & Habing 1988); as a sequence
of increasing initial masses (Epchtein et al. 1990); and as due to
the combined effects of increasing mass loss rate and also
increasing initial
stellar masses (Likkel 1990). VH show that the evolutionary track of most
of these stars in the [25-60] versus [12-25] colour-colour diagram is a
single function of [12-25] colour. This track passes through
regions II, IIIa,
IIIb and IV of VH. The envelope becomes thicker and cooler along
this track as
one moves from region II towards region IV; also there is an increase in
variability, thought to be due to simultaneous occurence of pulsation and mass
loss. However, there are other stars with CSE that populate a much wider area
of this colour-colour diagram; some of these are due to carbon
stars that have
cooler envelopes due to the higher emissivity of carbon dust in
their envelopes.
These populate the upper regions of IIa and region VII of the VH diagram.
Stars with a strong 
excess (corresponding to
cold dust) populate
regions VIa and VIb. This can arise from discontinuities in the mass loss
history of the star and due to the dust having moved farther
away from the star.
Protoplanetary Nebulae (PPN) and Planetary Nebulae (PN) are mostly found
in regions IV and V where stars with very cold CSE's are situated. Thus,
the VH diagram serves as a useful tool for obtaining preliminary information
on the evolutionary status of stars with circumstellar envelopes.
We present in Fig. 2 (click here) the programme sources on the VH diagram to
classify them according to their evolutionary status.