next previous
Up: BVRI photometry of

2. Observations

2.1. Optical identification

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

2.2. IRAS data on the 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 tex2html_wrap_inline1470 (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 tex2html_wrap_inline1472 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 tex2html_wrap_inline1474, except in the one case where it peaks at tex2html_wrap_inline1476 or beyond. The tex2html_wrap_inline1478 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 tex2html_wrap_inline1480 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 tex2html_wrap_inline1510 thirteen have LRS spectra. Only three of the sources in the group tex2html_wrap_inline1512 have LRS spectra. The IRAS data on the LRS spectral classification of these sources is presented in Table 2.

2.3. BVRI photometry of optical candidates of IRAS sources

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 (tex2html_wrap_inline1516), 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 tex2html_wrap_inline1520) covers a total field of tex2html_wrap_inline1522. 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 tex2html_wrap_inline1532, 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 tex2html_wrap_inline1590 refer to sources with tex2html_wrap_inline1592 and points tex2html_wrap_inline1594 refer to sources with tex2html_wrap_inline1596. 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) tex2html_wrap_inline1610 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

2.4. Classification of the programme sources according to their evolutionary status

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 tex2html_wrap_inline1630, and tex2html_wrap_inline1632 respectively, where the tex2html_wrap_inline1634 's are the IRAS flux densities at 60, 25, and 12 tex2html_wrap_inline1636m 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 tex2html_wrap_inline1644 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.

next previous
Up: BVRI photometry of

Copyright EDP Sciences