H Emission From Point Sources In The Magellanic Clouds

Past surveys

Table 1 is a list of the main H surveys of the Magellanic Clouds. All these surveys used objective-prism plates and most separated emission-line stars from nebulae according to whether a continuum could be seen adjacent to the H line.

Table 1: Magellanic Cloud H Surveys

Survey	Telescope	Gal	A		D	m	N	N
N,S	Mt Wilson 10in	LMC	225	620-670	450	13.5	172	415
N,S	Mt Wilson 10in	SMC	225	620-670	450	13.5	65	117
La	ADH Schmidt	SMC	50	590-670	800	15	804	109
Lb,LM,AL	ADH Schmidt	LMC	120	590-670	800	15	543	50
BE	Curtis ST	LMC	25	620-690	420	14.5	625	-
MA	Curtis ST	SMC	12	651-662	420	18	1755	143

Surveys: N,S: Henize (1956); La,b: Lindsay (1961,3); LM: Lindsay & Mullan (1963); AL: Andrews & Lindsay (1964); BE: Bohannan & Epps (1974); MA: Meyssonnier & Azzopardi (1993);
(Waveband) is in nm; D (Dispersion) is in Å/mm at H; m is the detection limit of the continuum in magnitudes; A is the survey area in sq.deg.

The most important early survey was by Henize (1956) which produced catalogues of H emission-line objects in the LMC/SMC containing 172/65 stars and 415/117 nebulae of which 26/38 % were star-like images. The later H surveys are progressively deeper through the use of better resolution emulsions and observational configurations. The most recent survey (MA) has quadrupled the numbers of known emission-line objects in the SMC. However, neither of the deeper surveys covers the 250/220 sq deg occupied by the Clouds and surveyed for [OIII]5007 emission from planetary nebulae (Morgan 1997).

The catalogues of objects identified in these surveys include most classes of bright emission-line object but do not distinguish between them. Although most of the brightest objects have been studied in detail and are some of the most interesting peculiar stars known, even today many of the fainter stars have not been observed spectroscopically, so these H catalogues may still include some important unclassified objects. For example, several symbiotic stars discovered on the basis of objective-prism spectra with HeII4686 emission and a red stellar continuum were found to be members of these catalogues (e.g. Morgan 1992).

One sample of 50 stars from the H catalogues was observed spectroscopically with FLAIR during its commisioning on the UKST. Of 36 previously unclassified objects (four from Henize (1956) and 32 from Bohannan & Epps (1974)), Morgan, Watson & Parker (1992) identified 15 Be stars including one Bep star, one HII region, one VV Cephei system, four red stars with H emission including one dM3-4e star, and 15 red stars with no apparent emission. Six of these red stars are galactic stars. The high percentage (36%) of stars with no H emission emphasises the need for secondary observational material when working in the spectral region around H where the spectra of red stars are by no means smooth.

Planetary Nebulae

The planetary nebulae are the most widely observed subgroup. Within the main surveys for planetary nebulae, the dominant method of discovery is identification of the [OIII]5007,4959 lines on an objective-prism plate. The high number of candidates rejected through follow-up spectroscopy (see review by Morgan 1997) shows that secondary plate material is important in eliminating spurious identifications from the initial candidate lists.

It is interesting to compare the results of the [OIII]5007-based UKST SMC Survey (Morgan 1995) with the H-based survey of Meyssonnier & Azzopardi (1993). The main difference lies in the large number of VLE (very low excitation) objects listed with the planetary nebulae in the latter survey. Many are previously identified VLE objects which, with some of the newly identified objects (Meyssonnier 1995), are known to have significant continua and are usually considered to be some kind of young emission nebula or peculiar emission-line star. So, excluding these, the H survey has 50 objects and the UKST survey has 39. Nine of the 11 absent from the latter survey have published slit spectra (Meyssonnier 1995). Seven of these have [OIII]5007/H < 2 and all nine except one are faint. The bright exception has [OIII]5007/H = 0.27 and although the remaining two have [OIII]5007/H > 2, they are very faint. It is therefore clear that some planetary nebulae of very low excitation are missed at the faint limit of [OIII]5007 searches. The loss here is 20%.

There is no deep H-based planetary nebula catalogue for the LMC, but the need for one can be assessed from Figure 1 which shows the line ratio [OIII]5007/H as a function of log(H flux) for those LMC planetary nebulae with published spectra and H fluxes. The source catalogues are based on detection of the [OIII]5007,4959 lines, and the figure shows an absence of low excitation nebulae from the fainter population. An H survey would fill this, as it did for the SMC.

Image temporarily unavailable Figure 1: Line ratio [OIII]5007/H versus log(H flux) for LMC planetary nebulae. The unit of flux is .

© Copyright Astronomical Society of Australia 1997