Subject: Re: Novae and other variables in LSST
From: Mike Shara
Submitted: Thu, 07 Aug 2003 20:11:01 -0400
Message number: 161
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Classical Novae as Tracers of Intergalactic Tramp Stars - Version #2
THE PROBLEM - 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 number ratio of stars
inside galaxies to intergalactic tramps.
CLASSICAL NOVAE - PHYSICS
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.
CLASSICAL NOVAE AS DISTANCE INDICATORS
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 or 3 magnitudes from that peak
brightness (Shara 1981 ApJ 243, 926). 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 tramp 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.
OTHER VARIABLES
A few-times-per-week all-sky survey to 24th magnitude will yield most of
the halo (and intergalactic) RR Lyrae stars as far away as M31; and
moderate and high galactic latitude contact binaries as far as the
Magellanic Clouds. These two populations' distinct light curves will
allow their easy identification with the
collection of ~100 or more observations (to unambiguously identify
periods for the variables). Distances are
readily determined for contact binaries as there is a tight
period-luminosity relationship. The spatial distributions
of these stars will be strong constraints on the
collision/stripping/cannibalism history of the Local Group.
A complete set of answers to the questions in LSST-102 FOR THE CLASSICAL
NOVAE is:
The area of sky imaged at any given time.
NOT IMPORTANT.
The total area of sky to be covered.
THE ENTIRE SKY, NIGHTLY, IS OPTIMAL. EVERY SECOND OR THIRD NIGHT IS
ACCEPTABLE. THE SAME 2,000 TO 4,000 DEGREES SQUARE IMAGED NIGHTLY FOR
AT LEAST 3 YEARS WOULD BE IMPORTANT TO SHOW THAT INTERGALACTIC TRAMP NOVAE
EXIST AND ARE USEFUL AS PROBES OF INTERGALACTIC STARS, BUT YIELD A SAMPLE TOO SMALL
FOR USEFULLY PROBING THESE OBJECTS' DISTRIBUTION.
The depth and dynamic range needed in a single exposure.
AT LEAST 22ND MAG, PREFERABLY 23RD MAGNITUDE
The depth and dynamic range needed in stacked exposure.
24TH MAGNITUDE
Length of individual exposures.
NOT IMPORTANT
Requirements on slew time.
NOT IMPORTANT
The requirements on seeing, PSF, and pixel size: uniformity of
PSF, aberrations of PSF, all as function of wavelength.
NONE ARE PARTICULARLY IMPORTANT, EXCEPT AS NEEDED
TO ACHIEVE S/N ~10 IN STACKED IMAGES
The filters needed
MILD PREFERENCE FOR B OR V
The need, if any, to stack the data.
ESSENTIAL TO GET AS DEEP AS POSSIBLE.
The photometric accuracy needed (both relative and absolute).
+/-0.1 MAGNITUDE RELATIVE AND ABSOLUTE AT 24TH MAGNITUDE IS SUFFICIENT
The astrometric accuracy needed (both relative and absolute).
+/-0.5 ARCSEC RELATIVE TO A NEARBY OFFSET STAR TO ENABLE FOLLOWUP, BLIND-OFFSET
SPECTROSCOPY.
Tails of the astrometric and photometric error distribution.
+/-1 ARCSEC AND +/-0.2 MAGNITUDE RELATIVE ERROR IS STILL ACCEPTABLE.
The cadence of observations needed (very different for moving
objects and, e.g., distant galaxies). Should the cadence
be dynamic?
NIGHTLY IS BEST, EVERY 2ND OR 3RD NIGHT ACCEPTABLE.
A SMALLER AREA NIGHTLY IS BETTER THAN A LARGE AREA WITH SPARSE COVERAGE.
Requirements of sky darkness and photometricity.
AS NEEDED TO REACH 24TH MAGNITUDE.
Requirements on the speed of data reduction needed, and the
nature of the measured quantities.
IDEALLY THE NEXT DAY, TO IDENTIFY CANDIDATE NOVA POSITIONS AND MAGNITUDES
Auxiliary data needed (e.g., follow-up spectroscopy,
observations at non-optical wavebands, and/or
a priori calibrating data)
FOLLOWUP OPTICAL LOW-RESOLUTION (5 ANGSTROM) SPECTROSCOPY AS SOON AS POSSIBLE
AFTER THE NOVA CANDIDATE IS DETECTED, TO CONFIRM THAT IT REALLY IS A CLASSICAL
NOVA IN ERUPTION.
Specialized data analysis tools needed to carry out the
science.
NONE. SIMPLY DIFFERENCE EACH NIGHT'S FRAME FROM A PREVIOUS NIGHT TO FIND
THE VARIABLES.
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