NEWS                      

Le supernovae non sono le uniche stelle di tipo esplosivo. Ci sono altre stelle che subiscono un improvviso aumento di luminosità e quindi sembrano comparire dal nulla nel cielo. Gli astronomi del passato pensavano che si trattasse di stelle appena nate, e diedero loro il nome di "novae", cioè "stelle nuove". In realtà, una nova è una stella piuttosto vecchia... Si tratta di una nana bianca che fa parte di un, cioè di una coppia di stelle. La compagna è una stella grande ed espansa, come una gigante rossa. Essa perde una parte del proprio gas, che dapprima si mette ad orbitare intorno alla nana bianca formando un disco, poi ne viene attratto e catturato. 

Man mano che cade sulla superficie della stella, il gas ne accresce la massa finché essa non raggiunge un certo limite. A quel punto, sulla nana bianca si accendono delle reazioni nucleari esplosive: essa lancia nello spazio la parte più esterna del proprio gas, nonché una gran quantità di radiazione ed energia.

Durante l'esplosione, la nova emette un'energia pari a quella emessa dal Sole in 100mila anni! Lo splendore della stella aumenta anche di decine di migliaia di volte.
L'esplosione di una nova è tuttavia molto meno violenta di quella di una supernova; inoltre essa non distrugge completamente la stella, ma solo i suoi strati più esterni.
Soltanto una piccolissima frazione della massa totale viene espulsa: dopo pochi anni, la stella torna quella di prima. Se la stella compagna continua a far cadere materia sulla nana bianca, il fenomeno può ripetersi anche più volte.

Anche le novae possiedono tutte la stessa luminosità nel momento del massimo splendore. Grazie a questo fatto, la loro distanza può essere calcolata semplicemente misurando la loro luminosità apparente.

La nova Cygni, nella costellazione del Cigno. La stella è esplosa nel febbraio del 1992; ciò che ne resta è l'anello di gas che vedi nella fotografia.

Electronic Telegram No. 593
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
CBAT@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/iau/cbat.html


NOVA IN M32
      D. C. Leonard, P. M. Nied, M. R. Davis, and A. W. Shafter, SanDiego State University; T. R. Norris, California State University,Chico; M. R. George, Harvard-Smithsonian Center for Astrophysics;
and A. Gal-Yam, California Institute of Technology, report on behalfof the Caltech Core-Collapse Project (cf. IAUC 8405) that a CCD spectrum (range 460-910 nm) of the apparent nova in M32 (cf. CBET 591), obtained on Aug. 2 UT with the double spectrograph mounted on the
Palomar Observatory 5-m Hale telescope, confirms that it is indeed a nova.  

Prominent H-alpha (FWHM = 1200 km/s) and H-beta, and weaker FeII, emission lines are visible in the spectrum.  The peaks of the slightly asymmetric Balmer lines indicate a velocity of recession of
about -180 km/s for the nova, strengthening the case for itsassociation with M32 (velocity of recession -200 km/s, from Huchra et al. 1999, Ap.J. Suppl. 121, 287).


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars.

                          (C) Copyright 2006 CBAT
2006 August 3      (CBET 593)          Daniel W. E. Green

La risposta da Monte Palomar del Dott. Douglas C. Leonard

Dear Federico --
  Thank you for providing your recent images of the nova in M32.  It has led me to look further into this event, both spectrally and photometrically.  The end result is that based on the spectrum and now the photometry, our group continues to believe this event to be a nova, although perhaps a somewhat unusual one.  There is some spectral similarity to a luminous blue variable (LBV) outburst, such as that which led to a misclassification as a supernova
  of SN 1997bs (see van Dyk et al. 1999 PASP, 111, 313), but the absolute brightness of the event is about 7 magnitudes too faint.  If you want to see all of the details, point your WEB browser to:
http://corelli.sdsu.edu/pickup/novam32.tar.gz

and you will get a whole lot of postscript plots that I have made: plot_lightcurve.ps : I did rough differential psf photometry on the fits images that you sent. It was difficult photometry, and so the results
    aren't perfect.  But, the bottom-line conclusion is that over 2 weeks, Nova M32 has remained as essentially the same brightness.  Please let me know if your lightcurve differs substantially from this.

  plot_novam32.ps    : Plot of the Nova in M32, spectrum from Palomar on 2006.08.02 
  plot_sn1997bs_19970416.ps : Plot of SN 1997bs, the LBV "supernova imposter".
  plot_m31_xxxxxxxxx.ps : Plots of 4 nova in M31 on various dates, spectraobtained by Filippenko.
  nova32_compare.ps : A comparison plot of SN 1997bs, an M31 nova from 1990, and the new M32 nova.

   All plots have had redshift removed.  Here's what we find:

        ** Spectra **
    Our Nova in M32 looks nearly identical to all 4 of the M31 nova which, in turn, look nearly identical to SN 1997bs.  The only difference that I can see is that SN 1997bs does not show an OI 7774 emission line, whereas
 our M32 nova and both M31 nova whose spectral range go out that far do. Otherwise, as I found before, SN 1997bs looks very much like any of these other novae. Thus, it would seem that the only real differentiation could come from the...

        ** Photometry **
   At the distance of M32, our Nova has M_V = -7.4 mag.  SN 1997bs had M_V = -14 at peak.  Galactic extinction in the direction of M32 is only E(B-V) = 0.062.  The H-alpha/H-beta flux ratio is 2.6, which is actually somewhat
 flatter than what one expects for case B recombination (2.8), so there is no evidence for excessive reddening from the emission lines. It was on this basis:  Luminosity consistent with a nova, and spectra
   consistent with a nova, that we announced this as a nova. The new wrinkle now is this somewhat unusual lightcurve behavior.  But, while most novae decline rapidly after peak, many do not (the so-called
   "slow" novae), and thus are not inconsistent with the present lightcurve. So, right now, we don't find reason to change the classification.  But, continued monitoring is certainly important, to see if it evolves in an unusual manner.

  Thanks again for sharing your images with us.

Best regards,

 Doug

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 Douglas C. Leonard                  Office: 619-594-2215
 San Diego State University          FAX   : 619-594-1413
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