Astronomical Data Analysis Software and Systems VII ASP Conference Series,Vol.145,1998R.Albrecht,R.N.Hook and H.A.Bushouse,e © Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Slitless Multiobject Spectroscopy with FOSC Type Instruments
V.F.Polcaro and R.Viotti
Istituto di Astrofisica Spaziale,CNR,Frascati,Italyitg
Abstract.We present a simple,effective technique that allows the con-temporary spectroscopy of all the objects,up to a limiting magnitude,contained in a ∼100arcmin 2field.This technique makes possible the immediate and unambiguous identification of peculiar objects included in the field.A data analysis procedure that produces the final results in a very short time has also been developed.As a test of this technique,we present the results ∗obtained for the peculiar young open cluster Berkeley
87.We show that this technique should also be quite useful in emission line and WR star surveys,and illustrate the software procedures devel-oped to make the wavelength and flux calibration of these spe
ctra.
(*)Based on data collected at the Loiano Observatory
Modern hard X-ray satellites allow the real time positioning of transient high energy sources with a precision of ∼10 –30 .In some cases,as for the gamma-ray bursts,the fast identification of the optical counterpart is a crucial item to unveil the physical nature of the emitting object.On the other hand,no other way to make their identification in the optical range has been so far found,but making the spectroscopy of all the objects included in the error box,and looking at the objects which show a peculiar (e.g emission line)optical spectrum.Of course,within a sky area of order of some 102arcmin 2,there is an enormous number of objects and it is unlikely that we can obtain all their spectra in a reasonable time period.This fact usually overcomplicates the identification of the optical counterpart of the high energy cosmic sources,at the risk of substantially diminishing the scientific potential of the imaging hard X-ray and gamma-ray telescopes.
For this reason,we have developed a technique,that allows the simultaneous spectroscopy of all the objects included in a square field of view with ∼10 side,using the widely used FOSC-type optical spectrometer and camera focal-plane instruments.The technique is based on the use of a slitless lo
太乐网w dispersion grism coupled with a broad-band filter,which perform an “objective prism like”spectrogram of all the objects present in the field,without serious overlap of adjacent spectra.For instance,the use of the very common grisms with ∼400-800nm bandpass and of the Johnson R filter will make an imaging spectroscopy of ∼200nm centered on H α,and emission line and peculiar spectrum objects are easily identified.
We tuned this technique on the Bologna Faint Object Spectrometer and Camera -BFOSC-instrument (Merighi et al.1994),mounted at the Cassegrain
78
Slitless Multiobject Spectroscopy with FOSC Type Instruments79 focus of the Bologna Astronomical Observatory“G.B.Cassini”1.52m telescope, sited near Loiano(Italy)at about800m altitude on the Appennine Mountains.
In order to test this technique,we used the peculiar open cluster Berkeley 87,an intriguing object,that we have studied for more ,Norci et al.1988;Polcaro et al.1989;Polcaro et al.1991;Manchanda et al.1996). Actually,most members are young,heavily reddened OB stars,but a few are much more evolved objects,such as the WO star Sand5and the M3.5I variable BC Cyg maki
ng the evolutionary status of the cluster extremely uncertain.The cluster member n.15in the Turner and Forbes(1982;hereafter TF82)list is an emission line star(V=11.8)also known as V439Cyg and MWC1015(Merril and Burwell,1949).This star dramatically changed its spectrum from late to early type in a few decades;furthermore,some absorption lines that were still present in1986and1987completely disappeared in1988.The star is characterized by a strong IR excess and a peculiar position in the HR diagram,suggesting that it should be considered as an intermediate RGS/LBV ,Polcaro and Norci,1997and references therein).Three other cluster members(no.3,no.9 and no.38in the TF82list)are emission line stars.
On June24,1997(23:11:55UT),we took a30s Rfilter exposure of the central(9x9arcmin2)region of Berkeley87,shortly followed(23:18:34UT)by a slitless300s image through the Rfilter and the grism no.4(Fig1).The Hαemission of stars no.3,9,15and38and the WR spectrum of Sand5where immediately visible after a short tuning of the contrast.The direct overlap of these two images using the IRAF task imarith on the recorded FITSfiles allowed a prompt and unambiguous correlation between the objects and the spectra.
The whole procedure,including the overlap,took less than half an hour.
Notice also that our short exposure allowed the immediate identification of the Hαemission of the V=14star no.38and of the WR spectrum Sand 5(V=13.8)without saturating the Hαemission of the V=7.81star no. 3.In absence of such a bright object(or if we do not care if we saturate its spectrum)a longer exposure should allow the identification of emissions also in much fainter objects.
A careful analysis of the slitless spectral image demonstrated that this tech-nique has afield of application much wider than the fast identification of peculiar objects described above.
全自动
电脑针织机Actually,after debiassing andflat-fielding(using standard IRAF proce-dures)by means of a10sflat-field taken through the Rfilter,we obtained a pretty clean image from which25good quality,not overlapped spectra of stars up to V=16were extracted.The comparison of this multiobject spectroscopy with long slit spectra of a few objects in the samefield taken on the same night through the same grism,which were wavelength calibrated by means of a He-Ar comparison lamp andflux calibrated using the standard star Kopf27,shows that the pixel-wavelength relationship was not affected by the lack of the slit and the presence of the photometricfilter,but just shifted in pixels,due to the different declination of the stars in thefield,and cut between550nm and780nm due to the combined effect of thefilter and grism transmission curves.Thus,the wavelength calibration is easily obtained,with a precision of±0.5nm from the position of the stellar image,the550nm cut-offand the red telluric absorptions.80Polcaro and Viotti
Figure1.Figure1:Slitless red spectral image of the open cluster巴黎公社原则
Berkeley87.Thefield is9x9arcmin2.North is at the top and East
to the left.The Hαemission line in star no.38(top left)and n.15(top
centre)and the WR spectrum of star n.29(Sand5,bottom centre)is企业
社交网络 l型匹配clearly visible.
A further benefit was obtained from the slitless technique:the night sky lines(and mainly the antropic lines that are quite strong at the telescope site, due to the close proximity of the city of Bologna)as well as the numerous nebular lines(mainly,but not only,Hα)due to the intracluster matter(, Polcaro et al.1991)result in a limited increase of the background level,so that spectra was much cleaner than those obtained in previous runs by means of slit spectroscopy.Furthermore,the absence of the slit eliminates the light loss due to the poor seeing(3.5arcsec in the night of our experiment)so that the signal-to-noise ratio was much higher than that of“classical”spectra.In this way we were able to unveil narrow Hαemission cores in the cluster members no.26,no.31and no.32.Furthermore,once the wavelength calibration of each individual spectrum was achieved,we recognized(from the comparison of
Slitless Multiobject Spectroscopy with FOSC Type Instruments81 the debiassed andflat-fielded slitless spectra in reduced CCD counts and of the long slitflux calibrated spectra of the same objects)that,due to the linearity of the CCD detector used in the BFOSC instrument,theflux calibration curve was the same for the whole image.Thus theflux calibration of the slitless spectra was also straightforward.
Our test demonstrates that the use of the slitless spectrometry technique with the modern FOSC type instruments does not only allow the immediate identification of spectroscopically peculiar objects in afield of the same order of magnitude of that of a“typical”hard X-ray instrument.It also shows how this technique can be used to obtain,without the need of either new hardware or of large telescopes or high quality skies,much valuable scientific information: for instance,we identified in a single short test-shot three previously unknown emission line stars.
We now plan to apply the method to a search for emission line and WR stars in young clusters.
We thank the night assistants of the Loiano observatory,Mr.G.Tessicini and Mr.E.Delogu,for their help during our experiment
References
Manchanda R.K.,Polcaro V.F., al.,1996,A&A,305,457
Merighi R.,Mignoli M.,Ciattaglia C.,et al.1994,“BFOSC User’s Manual”, RT09-1994-05,Bologna Astronomical Observatory.
Merrill,P.M.&Burwell,C.G.,1949,Astrophys.J.,110,387
Norci,L.,Giovannelli,F.,Polcaro,V.F.,&Rossi,C.1988,in“Frontier Objects in Astrophysics and Particle Physics”,F.Giovannelli and P.Mannocchi (eds.),Italian Physical Society,Bologna,Italy,7
Polcaro,V.F.,Giovannelli,F.,Norci,L.,&Rossi,C.,1989,Acta Astronomica, Vol389,No.4.
Polcaro V.F.,Rossi C., al.1991,Mem.S.A.It.62,933
Polcaro V.F.&Norci L.,1997,Proc.of the UMIST/CCP7Workshop on Dust and Molecules in Evolved Stars,Manchester,March24-27
Turner,D.G.&Forbes,D.,1982,Publ.Astr.Soc.Pac.,94,789(TF82).
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