• Volume 12, Issue 3

September 1991,   pages  179-268

• Integrated luminosity distribution of Galactic open clusters

The integrated magnitudes of 221 Galactic open clusters have been used to derive the luminosity function. The completeness of the data has also been discussed. In the luminosity distribution the maximum frequency of clusters occurs nearI (Mv) = −3m.5, and some plausible reasons for a sharp cut-off atI (Mv) = −2m. 0 have been discussed. It is concluded that the paucity of the clusters fainter thanI (Mv) = −2m.0 is not purely due to selection effects. The surface density of the clusters for different magnitude intervals has. been obtained using the completeness radius estimated from the logN- logd plots. A relation betweenI (Mv) and surface density has been obtained which yields a steeper slope than that obtained by van den Bergh & Lafontaine (1984).

• Study of faint young open clusters as tracers of spiral features in our galaxy - Paper 5: NGC 2236 (OC1 501)

Continuing the study of faint young open clusters as tracers of spiral features in our Galaxy, photoelectric and photographic photometry of 39 stars was done in the field of the faint open cluster NGC 2236 ≡ OCl 501 in the direction of Monoceros constellation. Out of these stars, a total of 22 down tomv ≃ 15.4 mag have been found to be probable members. There is apparently a variable extinction across the field of the cluster with E(B - V) ranging between 0.84 mag and 0.68 mag. The median age of this cluster is estimated to be 7.6 × 107 years and the cluster is thereby considered as belonging to the marginally old category. Thus, it cannot be specifically used as a spiral arm tracer in the study of our Galaxy. This cluster is located at a distance of 3.72 ± 0.13 kpc, which places it at the inner edge of the outer Perseus spiral feature of the Milky Way.

• Low-frequency observations of the Vela supernova remnant and their implications

We have studied the Vela supernova remnant in the light of the 34.5 MHz observations made with the GEETEE low frequency array. The flux densities of Vela X and YZ at 34.5 MHz are estimated to be 1800 and 3900 Jy respectively. These values, along with those from earlier observations at higher frequencies, imply spectral indices (S∞Να) of-0.16 ± 0.02 for Vela X and -0.53 ± 0.03 for Vela YZ. This situation is further substantiated by the spectral-index distribution over the region obtained between 34.5 and 408 MHz.

The spectral-index estimates, along with other known characteristics, strengthen the earlier hypothesis that Vela X is a plerion, while Vela YZ is a typical shell-type supernova remnant. We discuss the implications of this result.

• On the shell star pleione (BU Tauri)

BU Tauri (Pleione) an interesting star in the Pleiades cluster, has been observed spectrophotometrically. The energy distribution curves of the star have been discussed vis a vis model atmospheres for normal stars in the appropriate range of temperature and effective gravity. The changes in the energy distribution curve noticed during our observations and previous observations taken from the literature have been pointed out. On the basis of the measured Há emission equivalent width, a rough estimate of the dimensions of the extended envelope of the star has been made.

• Spot modelling and elements of the RS CVn eclipsing binary WY Cancri

Results of analysis of photoelectric observations of the RS CVn eclipsing binary WY Cancri in the standard passbands ofUBV during 1973-74, 1976-79 and inUBVRI during 1984-86 are reported. A preliminary analysis of the eclipses suggested the primary eclipse to be transit. A study of the percentage contribution of the distortion wave amplitudes in all the colours with respect to the luminosities of both components, showed the hotter component to be the source of the distortion wave. The clean (wave removed) light curves of different epochs have not merged, suggesting residual effects of spot activity. The reason for this is attributed to the presence of either (1) polar spots or (2) small spots uniformly distributed all over the surface of the hotter component. This additional variation is found to have a periodicity of about 50 years or more. The distortion waves in yellow colour are modelled according to Budding’s (1977) method. For getting the best fit of the observations and theory, it was found necessary to assume three or four spots on the surface of the hot component. Out of these four spot groups, three are found to have direct motion with migration periods of 1.01, 1.01 and 2.51 years while the fourth one has a retrograde motion with a migration period of 3.01 years. From these periods and the latitudes of the spots derived from the model a co-rotating latitude of 4ℴ is obtained. The temperatures of these spots are found to be lower than that of the photosphere by about 700ℴK to 800ℴK. Assuming the light curve of 1985-86, which is the brightest of all the observed seasons, to be least affected by the spots, the light curves of the other seasons are all brought up to the quadrature level of this season by applying suitable corrections. The merged curves in theUBVRI colours are analysed for the elements by the Wilson-Devinney method. This analysis yielded the following absolute elements:$$\begin{gathered} m_h = 0.86 \pm 0.03{\text{ }}M_ \odot \hfill \\ m_h = 0.51 \pm 0.03{\text{ }}M_ \odot \hfill \\ R_h = 0.99 \pm 0.02{\text{ }}R_ \odot \hfill \\ R_c = 0.65 \pm 0.02{\text{ }}R_ \odot \hfill \\ T_h = 5520^o K \pm 100^o K \hfill \\ T_c = 3740^o K \pm 20^o K \hfill \\ M_{h(bol)} = 4\mathop .\limits^m 96 \pm 0.10 \hfill \\ M_{c(bol)} = 7\mathop .\limits^m 58 \pm 0.15 \hfill \\ Spectral type hotter component = G5 \pm 1 \hfill \\ cooler component = K9 \pm 1 \hfill \\ \end{gathered}$$ For a mass ratio of 0.506 and with the derived fractional radii rh = 0.241 andrc = 0.157, both the components are found to be within their Roche lobes. Hence we have classified WY Cnc as a detached system. From their positions on the HR diagram it is concluded that both the components of WY Cnc belong to the main sequence.

• Spectroscopic binaries near the north galactic pole - Paper 20: HD 111068

Photoelectric radial-velocity measurements show that HD 111068 is a spectroscopic binary with a period of 206 days. The primary star is probably about type K5 III; the secondary, only detected through the photometric compositeness of the system, may well be an F dwarf. The orbit is circular within observational uncertainty; it is near the upper limit of periods for which tidal circularization operates for giant stars.

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