Southern Hα Index		VPHAS+ Contact		VPHAS+ Collaboration
SHS follow-up				Hα Index

SHS spectroscopic follow-up

This is one of the fields imaged in Hα, as part of the AAO UKST Southern Galactic Plane Survey (SHS). This is field 233, centred at 15.4 hrs, -56° (1950) in Norma. It spans a sky area of 4x4 square degrees.

The spectrum obtained per emission line point-source candidate, to date, has sampled the Hα region at a resolution of close to 2 Å (R ˜3000). These data have been gathered mainly using UKST/6dF. Targets were selected for spectroscopy from the imaging survey mainly on the basis of colour-colour diagrams in which apparent Hα 'excess' (obtained on comparing Hα-filter magnitudes with contemporaneous short-red magnitudes to give (R - Hα)) is plotted against broad-band (R - I) colour (the southern I band data also being available via the ROE SHS database.

To illustrate this approach at work, here are the results from an early success of the programme -- the discovery of a rare WO star, only the 4th to be found in the Milky Way. (A full download of the MNRAS paper on this star, named WR 93b, can be obtained here). At R = 14.7 this object is nearly 5 magnitudes brighter than the UKST Hα survey limit -- suggesting a lot of room for many more exciting discoveries.

First, below, a colour-colour diagram for a circular area of sky centred on the new WO star, 15 arcmin in diameter. Two objects stand out on the 'excess' side of a main locus of stars. The WO star is marked with an arrow. The other has yet to be followed up spectroscopically.

Figure: (R-Hα) versus (R-I) for point sources with 15 arcmin of the WO3 star, WR 93b. WR 93b is picked out by the arrow.

Figure: the discovery UKST/6dF 1516R spectrum of WR 93b

Lastly, below is the optical spectrum from 3700 to 7800 Angstroms made by combining WHT/ISIS blue-arm and red-arm spectra with a UKST/6dF 580V observation (bridging the ISIS dichroic gap). Note the enormously bright CIV 5801,12 Å emission line. This star has turned out to be exceptional in two respects: it currently holds the record for wind terminal speed (5750 km/s) among all non-degenerate stars and appears more enriched in carbon than any other Wolf-Rayet star (C/He = 0.95 by number).

Figure: merged optical spectrum of WR 93b. Relative flux calibration has been achieved, such that the obviously highly-reddened spectral slope is reliable.

We have also found a group of 5 WC9 stars in the same part of the Plane as the compact, massive OB association, Westerlund 1. These Wolf-Rayet stars lie within a few degrees with each other on the sky, around Galactic longitude 339, but are not physically associated. The paper describing these is here.

However Wolf-Rayet stars are not common among the emission line objects being found in this programme. Below are spectra of two less extreme emission line candidates, with more typical spectra. These were again obtained using the 1516R grating in UKST/6dF. Note that in both cases the Hα emission line width is very much narrower than in WR stars, but is still resolved by the grating.

Figure: 6dF/1516R spectra of a hitherto uncatalogued carbon star (m_R = 12.3, black line) and CPD -52° 9243 (m_R = 11.3, red line). The latter has been described as a 'low excitation P Cygni star' by McGregor et al (1988 ApJ 324 1071). Both are relatively bright objects in field 233, shown at the top of this page.

Our policy has been to aim for data of a quality that will reveal absorption line features useful for classification and a broad-brush impression of distance/luminosity (e.g. the strong diffuse IS band at 6284 Å falls in the observed range). The spectral resolution chosen is sufficient to reveal a meaningful intrinsic Hα line profile in most cases and to provide a measure of FWHM and radial velocity (where the profile is simple in form). A further advantage of this choice of resolving power is that it provides a sanity check for any incompletely subtracted nebular emission -- this will appear as an unresolved emission core superposed on what, in young and evolved objects, will be broader resolved emission.