![]() |
![]() |
Polcaro & Viotti (1998) proposed at the ADASS VII Conference a
simple slitless spectroscopic method that was effectively employed in
the search of very massive stars, as well as in the identification
of the optical counterparts of galactic X-ray sources (see e.g.
Bernabei & Polcaro 2001a,b; Israel et al. 1999).
In this regard, despite of the great
progress made in the last decade, the study of the very massive
stars remains hampered by the lack of statistics and
only a few hundred stars with M30 M
are known in our Galaxy. Actually, being short-living objects, the known very massive stars are associated with regions of large interstellar
extinction, hence they are not easy to identify in our Galaxy.
In fact, many the known objects of this category have been found
in external galaxies and in the Magellanic Clouds.
For instance, only one half of the LBVs known to date are
Galactic objects. On the other hand,
the new generation of optical telescopes makes the spectroscopy
of bright extragalactic stars a relatively easy job: therefore,
we can hope to increase our knowledge of very massive
objects from the study of this class of stars in nearby external
galaxies seen `face-on', that is with a little interstellar
matter interposed.
However, the problem arises in the preliminary identification
of the targets: very massive stars are difficult to identify
on the base of photometric surveys alone, being their colours
strongly affected by local and circumstellar reddening.
Unfortunately, the method that we have employed to date
for the search of galactic objects, is scarcely useful,
being based on a combination of filters and grisms,
suitable only for relatively bright objects (our sensitivity
limits is of the order of V
14).
Because of these reasons, we developed a new slitless procedure,
allowing the spectroscopic survey of a relatively wide sky region
on a single image, using the current observatory's
instrumental set-up.
As a test of the procedure we have obtained a slitless image
of a 1313 arcmin
region around the well studied
galaxy M 101 using the Loiano Observatory 1.52 cm telescope
equipped with the Bologna Faint Objects Spectrometer and Camera
(BFOSC, Gualandi & Merighi 2001), and
an EEV D129915 CCD (1300
1340 pixels).
A grism (no.3, with a resolution of 5.5 Å) selects a wavelength
window (nominal bandpass: 3300-6420 Å) centred near H, where the
most characteristic spectral features of bright hot stars,
Wolf-Rayet stars and
Luminous Blue Variables (LBV) are present, as well as those of
nebular H II regions and AGN's.
The BFOSC unfiltered image of M 101 is shown on the top of Fig. 1,
and on the bottom the corresponding 60 sec slitless spectral image,
with the central wavelength (about 4900 Å) coincident with
the stellar image, so that the target identification of the
`strips' in the spectral image is straightforward.
Notice that the actual recorded wavelength range depends on
the y-position (i.e. declination) of the object in the field.
Hence, the H
region could be present in the spectra
of the objects sited in the upper part of the field (e.g. NGC 5455)
but is absent in the more northern objects,
as it is the case of NGC 5461.
The 60 sec spectral image of M 101 (bottom of Fig. 1) shows many
`spectral strips' corresponding to different regions of M 101,
the most intense ones are the central core of the galaxy,
a compact region
7 arcmin to the SE, and two
H II regions SW and SE of M 101: NGC 5455 and NCG 5461.
The extracted spectra of the two H II regions are
shown in Fig. 2 in order to illustrate the data quality that can
be obtained from this procedure also with a short exposure time.
They are characterized
by strong nebular emission lines of hydrogen (H
, H
,
H
), and of [O III] (5007, 4959 and 4363 Å).
The H
-[O III] signature of the two regions
is easily seen in the spectral image.
In principle, these lines together with the 4640-4686 Å
feature, can be used for a quick-look identification
of emission-line objects in the field,
such as Of, WR, LBV stars, and compact H II regions
(e.g., D'Odorico et al. 1983).
Diffuse emission is also present in the spectrum of NGC 5455,
probably from extended diffuse nebulosity, but the very short
exposure time does not allow a more detailed analysis of
this feature.
Finally, we recall that this method not only permits the object identification in the field, but also an accurate spectrophotometric investigation of the many objects present in the image, by using photometric standards in the same or in a nearby field. This will be the next step of our work.
Bernabei, S. & Polcaro, V. F., 2001a, A&A, 366, 817
Bernabei, S. & Polcaro, V. F. 2001b, A&A, 371, 123
Gualandi, R. & Merighi, R., 2001, BFOSC - Bologna Faint Object Spectrograph & Camera, MANUALE UTENTE Rel 2.0 (in Italian) R.T.25-03-2001
Israel, G. L, Covino, S., Polcaro, V. F. & Stella, L. 1999, A&A, 345, L1
D'Odorico, S., Rosa, M. & Wampler, E. J. 1983, A&AS, 53, 97
Polcaro, V. F. & Viotti R. 1998, in ASP Conf. Ser., Vol. 145, Astronomical Data Analysis Software and Systems VII, ed. R. Albrecht, R. N. Hook, & H. A. Bushouse (San Francisco: ASP), 145, 78