Variable Stars clues: timescale, amplitude, light curve shape, spectrum Eclipsing: Algol ß Lyr W...
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Transcript of Variable Stars clues: timescale, amplitude, light curve shape, spectrum Eclipsing: Algol ß Lyr W...
Variable Stars clues: timescale, amplitude, light curve shape, spectrum
Eclipsing: Algol ß Lyr W UMa B8-M (hrs-days) B8-G3 F0-K0 (hrs)
Eruptive: single binarySNII 15-20 mag (yrs) flare 1-6 mag (<hr) K-M
WD: SNI -20mag (yrs) N -10mag (1000s yrs) DN - 2-7 mag (weeks) NL - erraticSymbiotic: 3mag (erratic)XRB: HMXRB, LMXRB-ray BurstersRS CVn: F,G+KIV, spots
Pulsating: short P long P oddCepheids:F-K, 1-50d, 1.5mag
RR Lyr: A-F, 0.5 day, 1 mag
Scuti: A-F, hrs, 0.02 mag
Mira:M, yrs, 1-5magS-R: K, M
ß Ceph: B, 0.5dZZ Ceti: WD, min
Cataclysmic Variableswhite dwarf primary with a low mass (G-M) secondary, orbital periods of 67 min-2 days
Nova: TNR, high mass WD, outbursts 8-15 mag every few thousand yrs, ~20/yr in MW
Dwarf nova: disk instability, outbursts 2-7 mag every week-30 yrs
Novalike: high, low states on timescales of months, high accretion
AM CVn: 2 white dwarfs, orbital periods of 10-45 min
DISK ACCRETION MAGNETIC
High M Low M. .
X-rays
108 K
9000-40000 K
ACCRETION
BL
For slowly rotating WD:
Ldisk = LBL = 1/2GMMwd/Rwd
.
Hard X-rays
Soft X-rays
Cyclotron
-WD accretes until thick degenerate envelope leads to TNR
-10-6 to 10-4 solar masses ejected (dust can form)
-Envelope returns to quasi-static equil (nuclear burning)
-Envelopoe gradually ejected by winds, phot recedes, T increases and peak radiaiton in soft X-ray
-Constant bol phase lasts until envelope exhausted (1-100 yrs)
Outburst cycle of the Dwarf Nova SS Cyg
Cannizzo & Mattei, 1998, ApJ 505, 344
Outbursts are DISK instabilities
Typical DN
Symbiotic stars
Late type RG +
WD +
high excitation emission lines
Possible progenitors of Type Ia SN?
Pulsating stars:Asteroseismology
• Pulsations Only systematic way to study the stellar interior
• Pulsations are observed in stars all over the HR diagram ZZ Ceti stars
Pulsations in a star
Pulsation period and amplitude depend onthe average density. P 1/2 [P=(4π/3G)1/2]Low density long P, high amplitudeHigh density short P, low amplitude
Density profile decides how deep the pulsations penetrate in the star.
(Deeper the penetration more we learn about the interior)
Centrally condensed stars like our Sun have shallow pulsations
Uniform density stars like white dwarfs have deep pulsations
Cepheids and RR Lyrae
Cepheids: F-G SG, P-L relation, P ~ 2-60 days,
HeII ionization zone pulsation mechanism
RR Lyrae: A giants, Mv = 0.5, P<1 day
Radial pulsators
• White dwarfs show non-radial g-modes on account of their high gravity
Periods of 100s to 1000s
• These modes are characterized by quantum numbers (k,l,m)
similar to atomic orbitals
Spherical gravitational potential Spherical electrostatic potential
l determines the number of borders between hot and cool zones on the surface m is the number of borders that pass through the pole of the rotation axisk determines the number of times the pulsation wiggles from the center to the surface
Two flavors of ZZ Ceti stars (DAVs)
Teff = 11000K P ~ 1000s
Teff = 12000K P ~ 200s
cool
Larger amp, more modes, unstable amps
hot
Less modes, more stability
Flare Stars
Flare <15s to 1 hr, repeats hrs - days
Amplitude up to 4 mag
Opt is thermal brem at T ~ 107K, radio is non-thermal
Between flares, spectrum is K-M with CaII, H emission
Variable Stars clues: timescale, amplitude, light curve shape, spectrum
Eclipsing: Algol ß Lyr W UMa B8-M (hrs-days) B8-G3 F0-K0 (hrs)
Eruptive: single binarySNII 15-20 mag (yrs) flare 1-6 mag (<hr) K-M
WD: SNI -20mag (yrs) N -10mag (1000s yrs) DN - 2-7 mag (weeks) NL - erraticSymbiotic: 3mag (erratic)XRB: HMXRB, LMXRB-ray BurstersRS CVn: F,G+KIV, spots
Pulsating: short P long P oddCepheids:F-K, 1-50d, 1.5mag
RR Lyr: A-F, 0.5 day, 1 mag
Scuti: A-F, hrs, 0.02 mag
Mira:M, yrs, 1-5magS-R: K, M
ß Ceph: B, 0.5dZZ Ceti: WD, min