the spectrum of supernova 1987a · 2010. 3. 24. · figure 2: spectrum of 1987a, obtained by r....

The Spectrum of Supernova 1987A 10340 6961. 911 6003.1152 8630 610 911 004 .052 other, more subtle, changes as broad spectral features appear and fade as the ionization conditions in the atmosphere change. Because they are so broad, 6920 5251. 911 51 1152 LA SILLA 3. GM ZMAR.. 1987 UT ESO 3. 614 LA 5I LU Z7-FEB-87 33 1Il216 3483.1151 4 .052 WAVELENGTH CA) *0096 SNI987A R+8 3S00 52111 WAVELENGTHCA) "" "" "" ...: are also becoming stronger as the vol- ume of the emitting gas increases and the temperature of the absorbing col- umn decreases (Fig. 1 b). There are Figure 1: Spectra of 1987A, obtained by J. Oanziger and R. Fosbury al Ihe ESO 3.6-m telescope with the Boiler and Chivens spectrograph. The upper spectrum (a) was obtained on February 27.05, four days after the explosion and already shows the strong P Cyg profiles mentioned in. the text. The lower spectrum (b), although qualitatively similar, displays importanl changes in the amplitude and also in the velocities of the main features. "" • .::.6M:....2::.M::;:AR.::.. •• :.:I.::.987::.:.....:U::.:.T,, -,-__-, < o <. -l I -l L.!- W :::l:j I- . < -l W IX Figure 2: Spectrum of 1987A, obtained by R. Fosbury (3.6-m + B & C spectrograph + RCA CCO) on March 2.1 and covering the 350-950 nm spectral region. In the early phase of a supernova, the dense atmosphere of the progenitor star is blasted off with a high velocity. This surface radiates like a "black-body" and, as such emits a continuous spectrum wh ich is essentially independent of the chemical composition and whose energy distribution is determined solely by its temperature: the light is radiated by charged particles as they scatter off one another. As the "atmosphere" grows in size, the range of depths from which the photons can escape increases and the chemical composition becomes impor- tant in determining the opacity of the gas at different wavelengths. This is when spectral lines start to become ap- parent. But still, and indeed for a long time after maximum light, most of the energy is radiated as an approximately black-body continuum with a temperature - at least in type 11 supernovae, which are supposed to be hydrogen rich - corresponding to the layer where the hydrogen becomes fully ionized. The lines just im pose a modulation on this underlying continuum. The outer part of the atmosphere, weil above the continuum emitting surface, will radiate emission lines while that part between the surface and us will absorb the same lines. Since the atmosphere is expanding, the absorbing part will have the greatest component of velocity towards us and so will produce "blue-shifted" lines. Such an emission/absorption structure is known as a "P-Cygni" profile after the prototype Be star with an expanding atmosphere. Measurements of the positions (in wavelength), strengths and shapes of such profiles provide as- tronomers with the means to study the composition and the velocity evolution of the expanding shell. The first optical spectra of SN 1987 A obtained at ESO on the night of 24-25 February show very weak spectral features and an overall energy distribution which corresponds to that of a hot star. Over the next two days, strong, broad spectral features began to appear and by 27 February (Fig. 1 a), strong P-Cygni lines are seen which correspond primar- ily to the Balmer se ries of hydrogen. The shift of the H-alpha absorption trough with respect to the rest wavelength corresponds to a velocity of over 17,000 km/s when the first measurements were made. The spectrum now appears to be evolving in the sense that the overall energy distribution is becoming redder (a cooling of the continuum emitting "photosphere") and the velocity shifts of the P-Cygni absorption compo- nents are slowly decreasing. The lines 32

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The Spectrum of Supernova 1987A

10340

6961. 911

6003.1152

8630

610 911

004 .052

other, more subtle, changes as broadspectral features appear and fade as theionization conditions in the atmospherechange. Because they are so broad,

6920

5251. 911

51 1152

LA SILLA 3. GM ZMAR.. 1987 UT

ESO 3. 614 LA 5ILU Z7-FEB-87

33

1Il216

3483.1151 4 .052WAVELENGTH CA)

*0096 SNI987A R+8

3S00 52111WAVELENGTHCA)

""""""

...:

are also becoming stronger as the vol-ume of the emitting gas increases andthe temperature of the absorbing col-umn decreases (Fig. 1b). There are

Figure 1: Spectra of 1987A, obtained by J. Oanziger and R. Fosbury al Ihe ESO 3.6-mtelescope with the Boiler and Chivens spectrograph. The upper spectrum (a) was obtained onFebruary 27.05, four days after the explosion and already shows the strong P Cyg profilesmentioned in. the text. The lower spectrum (b), although qualitatively similar, displays importanlchanges in the amplitude and also in the velocities of the main features.

"" ,--:..:*1il09=-4:--=SN::.:.1;.:98;:..:7.:.:.A-=0~11-=2=-5.;..UT:.......,.._--,-_,....::LA.:...::.:SI-=LL::.A~3::.• .::.6M:....2::.M::;:AR.::..••:.:I.::.987::.:.....:U::.:.T,, -,-__-,

~
these features are notoriously difficult toattribute to different chemical elementsbut it appears likely that the strongest ofthem can be identified with singlyionized calcium and iron. When the opti-cal and infrared data are combined, thetemperature of the "black-body" con-tinuum is readily measured; it was5900 K on the night of 1-2 March (seethe plot of the infrared - optical photo-metry). The fact that this temperature

was higher in the initial phase was goodnews for the IUE for wh ich the superno-va now provides a greatly weakenedsource.

Another aspect of the spectroscopywhich is causing great excitement is thepossibility such a bright object in theLMC provides for the study of the inter-stellar/intergalactic medium between usand the supernova. Sight lines outsideour own Galaxy have been studied be-

fore, using as background sourcesbright stars in the Magellanic Cloudsand much more distant Quasars andSeyfert galaxies. These, however, arevery faint objects and the opportunitypresented by a naked eye supernovahas already resulted in a Bonanza ofresults from the very high resolutionspectrograph (CAT/CES) at ESO (seecontribution by Andreani, Ferlet andVidal-Madjar). R. Fosbury (ST-ECF)

High-resolution Spectroscopy of 1987A