An analysis of the composite spectrum of the Large Magellanic Cloud.
Abstract
We have studied a low-dispersion spectrum, 2ioA/mm at H-K, of the brightest part of the bar of the Large Magellanic Cloud (5h20~, 69035?) secured in December 1956 with the 2-prism Zeiss spectrograph at the Newtonian focus of the Mount Stromlo 7~-inch reflector (UV optics, f/I.5 camera, expos~ire 15 hours, emulsion Io3aO, range 3500A-4900A). The region selected is substantially free of supergiant stars of m ~ 14 and of ThIl regions; apart from the weak superimposed emission bands of the airglow the only emission line originating in the Cloud is the usual XX3727-9 of EOIIJ with moderate strength. The stronger absorption lines attributable to the Cloud are Hp, 4582 (blend), 4556 (blend), 4384-8 (FeI +HeI?), H~, the G band, 4226 (Cai), 4200 (blend), 4144 (Fei + Hei?), 4032 EMni), 4028 (Hei), H + He, K, H~, H9, 3820 (Hei), HIo, ThIl, H12, H13?. The equivalent widths of the more important lines were measured, or estimated, as follows: Hp: (3.0), H~: 5.35, H~: 7.3, H + He: 9.25, H~: (3.65)*, H~: (4.75)*, 3820: 0.85, Hio: 5.8,Hii: (4.0), ThI2: (I.8)*. The "equivalent" spectral type varies rapidly with wave length: XX37oo-38oo: Bo-B5, XX38oo-39oo: B5-Ao, XX39oo~4ooo: Ao~A5, XX4ooo-4Ioo: A5-Fo, ~X42o&~44oo: Fo-F5, XX46oo-49oo: F~-Go. An attempt was made to determine the relative contributions to the visual luminosity of the region of stars belonging to each of the main spectral types so as to fit both the observed line * Partly filled-in by 3888, 3873, 3834 and 3741-54 emission of airglow. widths and the measured color index B - V = +0.49. Starting w:th a van Rhijn distribution for the Galaxy near the sun, the following distribution was found by trial and error: Model Sp. per cent van Rhijn 0 1 0.1 B 33 11.4 A 7 21.2 F 5 21.2 G 10 17.3 K 30 24.9 M 14 3.9 B-V +0.50 - U-B -0.35 - The main conclusion is that there is a large excess of OB and KM giants in the core of the Cloud compared with the Galaxy near the sun and a corresponding deficiency in types A, F, G. This agrees with the results of early Harvard spectral surveys disclosing an excess of OB and KM supergiants in the Cloud and is consistent with current ideas on stellar evolution. Lowell Observatory, Flagstaff, Aris.
- Publication:
-
The Astronomical Journal
- Pub Date:
- February 1958
- DOI:
- 10.1086/107663
- Bibcode:
- 1958AJ.....63Q.304D