CBA Center for Backyard Astrophysics



News

recent · all
2009 · 2008 · 2007 · 2006 · 2005 · 2004 · 2003
2002 · 2001 · 2000 · 1999 · 1998 · 1997 · 1996

    Re: [Fwd: Re: [baavss-alert] V358 Lyr]

    From: Joe Patterson <jop_at_astro.columbia.edu>
    Date: Mon, 24 Nov 2008 10:43:50 -0500
    Hi Berto et al.,
    
    Well, 8 magnitudes is impressive... but not shocking.  Just in one 
    constellation, there's PQ And, V455 And, V466 And:  all around 8 
    magnitudes, and all plainly accretion-style events with Mv near +4 at 
    the peak of the outburst.  Even WZ Sge, the granddaddy and defining star 
    of the class, is at 8.0 mag range if you count the very peak of 
    outburst... and certainly has Mv near +4 at the peak.  All are obvious 
    dwarf novae.
    
    Some of the OBVIOUS recurrent novae (e.g. T CrB) also have a range of 8 
    mag.  Doesn't this destroy the old classification of (supernova, 
    recurrent nova, dwarf nova) by magnitude range - made famous in many 
    books and papers?
    
    Yes, it does.  Without any doubt.  We now classify these things by the 
    physical mechanism involved: thermonuclear runaway in a "recurrent 
    nova", disk instability or some similar accretion event in a "dwarf 
    nova".  The former is typically at Mv=-6 to -10, the latter at +4 to +5.
    Of course, distances can be difficult to measure, and certain 
    identification of the underlying physics much more so... so this 
    classification scheme is not ideal for the observer!  But a spectrum is 
    usually decisive: an erupting dwarf nova is pretty much a light bulb, 
    and a recurrent nova is much like a classical nova (except that it 
    recurs in historical memory), with very bright emission lines.
    
    The physical reason that *range* is confusing is this: in order to 
    supply enough mass to trigger a classical nova outburst in a few 
    decades, the binary's donor star must lose matter at a very high rate.
    And that means it must be a pretty luminous star - usually a red giant 
    with Mv~0.  Thus, eruption to -8 gives a range of 8 magnitudes or so.
    
    So a spectrum at quiescence will tell the story too.  You'll usually get 
    a red giant (8 mag fainter than -8), or something like a white dwarf (8 
    mag fainter than +4).
    
    There are some weird stars, like T Pyx, that don't fit this dichotomy... 
    but most clearly join club or the other.  For V358 Lyr, put your 
    proverbial money on dwarf nova - pretty far away!
    
    joe
    
    Received on 24 Nov 2008