Variability

Learning outcomes

• Variability classes of long period variables

• Various time scales of variability phenomena

• Spectroscopic variability

• Connection between variability and atmospheric structure

Mira – the wonderful star

Historical Background

• Catalog der rothen, isolirten Sterne, welche bis zum Jahre 1866 bekannt gevorden sind. Von Herrn Professor, Dr. Schjellerup

• 1782 observation of semiregular variability of µ Cep by Sir William Herschel

Variability class: Mira

Mira (Omicron) Ceti-type variables. These are long-period variable giants with characteristic late-type emission spectra (Me, Ce, Se) and light amplitudes from 2.5 to 11 mag in V. Their periodicity is well pronounced, and the periods lie in the range between 80 and 1000 days. Infrared amplitudes are usually less than in the visible and may be <2.5 mag. For example, in the K band they usually do not exceed 0.9 mag. If the amplitudes exceed 1 - 1.5 mag , but it is not certain that the true light amplitude exceeds 2.5 mag, the symbol "M" is followed by a colon, or the star is attributed to the semiregular class with a colon following the symbol for that type (SR).

Variability class: SRV

SRA Semiregular late-type (M, C, S or Me, Ce, Se) giants displaying persistent periodicity and usually small (<2.5 mag in V) light amplitudes (Z Aqr). Amplitudes and light-curve shapes generally vary and periods are in the range of 35-1200 days. Many of these stars differ from Miras only by showing smaller light amplitudes;

SRB Semiregular late-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20 to 2300 days) or with alternating intervals of periodic and slow irregular changes, and even with light constancy intervals (RR CrB, AF Cyg). Every star of this type may usually be assigned a certain mean period (cycle), which is the value given in the Catalogue. In a number of cases, the simultaneous presence of two or more periods of light variation is observed;

SRV lightcurvesK

ersc

hbau

m e

t al.

2003

Variability class: SRV

SRC Semiregular late-type (M, C, S or Me, Ce, Se) supergiants (Mu Cep) with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days;

SRD Semiregular variable giants and supergiants of F, G, or K spectral types, sometimes with emission lines in their spectra. Amplitudes of light variation are in the range from 0.1 to 4 mag, and the range of periods is from 30 to 1100 days (SX Her, SV UMa).

Variability class: L

LB Slow irregular variables of late spectral types (K, M, C, S); as a rule, they are giants (CO Cyg). This type is also ascribed, in the GCVS, to slow red irregular variables in the case of unknown spectral types and luminosities.

LC Irregular variable supergiants of late spectral types having amplitudes of about 1 mag in V (TZ Cas).

stars are often attributed to this type because of being insufficiently studied

Variability on various time scales

• Short time variations

• Humps

• Long secondary periods

• Period changes

Outbursts in miras

de Laverny et al. 1998

Long secondary periods

Lebzelter &Hinkle 2002

Bedding et al. 2002

L2 Pup

Secular evolution

Tempelton et al. 2005

Colour variations 1

Le Bertre 1993Le Bertre 1992

Colour variations 2

Whitelock et al. 2000

Colour variations 3

Bouchet 1984

Spectral variability

• Strong changes in the shape of visual spectra with pulsation phase

• TiO-bands dominant feature – sensitive to temperature changes

• IR-spectrum: mainly changes due to H2O

• Occurrence of emission lines

• Radial velocity variations

Aringer et al. 2002

H2O CO2

Max

Min

Min

~Max

Emission lines

• in miras and some semiregular variables

• present over 2/3 of the light cycle (not present after minimum)

• typically brightest after maximum

• first H lines, later Fe II, Fe I, Mg I, Si I, Mn II, Cr II, Ca II, Sr II, ...

• in some stars: He I 10830 Å

Fox et al. 1984

multiple emission or self-absorption?

Joy 1954

Hinkle, Hall & Ridgway 1982

Near-IR absorption lines

Hinkle, Lebzelter & Scharlach 1997

Alvarez et al. 2000

Nowotny et al. 2005

Lebzelter et al. 2005

Lebzelter et al. 2001

Lebzelter et al. 2001

Wallerstein 1985

Na I, K ICr I, Mn I

TiO?

400 nm

800 nm

800 nm

CO linesv=1v=1

low exc.v=2

mod.exc. v=2

high exc. v=2v=3

high exc. v=2v=3

Variability and Parallax

Bastian & Hefele 2005

Filin 1952

Microlensing surveys and LPVs

• MACHO: http://wwwmacho.mcmaster.ca/

• OGLE: http://www.astrouw.edu.pl/~ogle/

• EROS: http://eros.in2p3.fr/

• MOA: http://www.phys.canterbury.ac.nz/moa/