Subject: novae

From: Mike Shara

Submitted: Tue, 15 Apr 2003 19:11:05 -0400

Message number: 107 (previous: 106, next: 108 up: Index)

 Classical Novae as Tracers of Intergalactic Tramp Stars

Galactic cannibalism and harassment is a process critical to 
understanding the evolution of galaxies. Over a Hubble time many 
(perhaps most) galaxies suffer
one or more close encounters with other galaxies. The outcomes for one 
or both galaxies range from benign perturbations to catastrophic 
disruption. Tidal tails are often observed in colliding galaxies. 
Simulations reproduce these features amazingly well, and demonstrate 
that many stars are liberated from galaxies during collisions. The 
presence of such "intergalactic tramps" is supported observationally via 
the detection of red giants and planetary nebulae, in the Virgo and 
Fornax clusters of galaxies. Our knowledge of tramps outside these 
systems is essentially nil. Are they common or rare? Novae and LSST 
offer the opportunity to directly determine the ratio of stars inside 
galaxies to intergalactic tramps.

Roughly once per decade a Galactic classical nova attains naked eye 
brightness.
For a few days or weeks the object shines with 10**5 Lsun, before fading 
back to
15-20th magnitude obscurity. The Milky Way is host to roughly 20 
classical novae every year,(as is M31), though only a few are close and 
bright enough to be detected...often by amateurs.

The physical processes underlying classical nova explosions are 
extremely well understood. A white dwarf accretes hydrogen-rich matter 
from a main sequence companion, developing an electron degenerate 
envelope of roughly 10**-5 Msun and 1 km in depth. The pressure at the 
base of the hydrogen-rich envelope eventually becomes large enough to 
initiate nuclear fusion. This turns into a thermonuclear runaway because 
pressure does not rise until temperatures in the electron degenerate 
matter exceed 10**8  Kelvin. The resulting rapid nuclear energy release, 
visual luminosity rise and envelope ejection produces a classical nova.

Novae, it turns out, are excellent standard candles. There is a tight 
and (theoretically) well understood relationship between observed nova 
peak luminosity and time to decline by 2 magnitudes from that peak 
brightness. This absolute magnitude-decline time relationship (which is 
fundamentally due to
the tight mass-radius relationship for all white dwarfs) has remarkably 
small scatter
and is independent of underlying binary population metallicity. Novae 
are seen in very old populations (eg in the giant elliptical galaxy M87 
and in the globular cluster M80) so they are clearly long-lived. For all 
these reasons novae are potentially superb tracers of intergalactic 
tramp stars.

About half of all novae are bright enough (M< -7) for long enough (1-2 
weeks) to be detectable by LSST out to distances of about (m-M) ~ 31 
...ie out to the Virgo or Fornax clusters of galaxies. There are roughly 
one hundred galaxies with masses comparable to the Milky Way or M31 out 
to this distance, each displaying about 20 novae annually... roughly 
2,000 novae/year in galaxies accessible to LSST. If a (very) 
conservative 10% of all stars out to (m-M) ~ 31 have been ripped from 
these galaxies then roughly 200 intergalactic tramp novae will be seen 
every year by LSST. The ratio of tramp to galactic novae should mirror 
that of tramp to galactic stars. The ~1000 novae detected during a 5 
year LSST survey will have well determined distances and thus will act 
as probes of the spatial distribution of all intergalactic tramp stars.

While an ideal observing campaign would image the same piece of sky 
every night to
catch every extragalactic nova at its peak brightness, observations 
every second or third night will still yield light curves complete 
enough for very good distance determinations. Novae typically display 
B-V ~ 0 near maximum light, and the light curves in these two passbands 
are particularly well calibrated, so there is a modest
preference for these filters.


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