Mass loss from stars

Transcription

1 Mass loss from stars Can significantly affect a star s evolution, since the mass is such a critical parameter (e.g., L ~ M 4 ) Material ejected into interstellar medium (ISM) may be nuclear-processed: changes in chemical abundances for future generations of stars Stellar winds shape ISM: Spitzer Space Telescope images, GLIMPSE survey shows bubbles from fluorescing dust in the infrared

2 RCW 79 JHK (2MASS)

3 RCW

4 G JHK (2MASS)

5 G Ch 1,2,3

6 MASS

7 Ch 1,2,4

8 MASS

9 Ch 1,3,4

10 RCW 49 JHK (2MASS)

11 RCW

12 Detecting Stellar Winds and Extended Atmospheres Stars with normal atmospheres Stars with extended atmospheres Star emission E 1 Star emission E 2 continuum level Normal spectrum: continuum + absorption lines absorption line profile abs. A E 1 E 2 slow rotator faster rotator A λ 0 increasing λ

13 Expanding atmospheres Stars with expanding atmospheres Stars with thick expanding atmospheres D D C Star C B B A C D B A λ 0 increasing λ P Cygni profile : determine wind speed from Doppler shift of blue edge of absorption trough λ 0 increasing λ

14 Mass Loss Rates dm dt = 4π r 2 ρ(r)v(r) ρ(r), v(r) are density and speed of stellar wind cf photon flux & luminosity: L(r) = 4πr 2 F(r) (ρ v) is mass flux: kg m -2 s -1, dm/dt is mass loss rate: kg s -1 (M sun year -1 ) Determine v from P-Cygni line profiles Determine ρ from spectral line analysis Determine r: difficult, but can for resolved winds

15 Which stars lose mass and how? 1. ALL O stars: M V = -4 to -7 and T eff > 30,000 K => high radiation pressure, esp. in UV Show P Cygni profiles in UV lines of highly-ionized atoms, e.g. NV, CIV, SiIV 2. Wolf-Rayet stars = evolved massive stars: have thick atmospheres expanding at speeds ~ 2000 km s 1 show chemical peculiarities: high abundances of C, N from nuclear processing. 3. Red giants and supergiants: show winds expanding at ~ 20 km s 1 with dust being formed at large distances from them.

16 4. Planetary nebula stage Emission lines of H, He etc. P Cygni profiles showing ejection of envelopes at ~ 50 km s 1 Some PNe show several shells with different chemical abundances e.g. Abell 58 : Old H-rich envelope of few thousand years ago + recent ejection (1910) with zero H, plenty He real-time stellar evolution!

17 5. Supernovae More than J are imparted to several solar masses in a VERY short time SUPERNOVA! Most of the star is ejected into interstellar space. Expansion velocities 2000 to 10,000 km s 1. Star brightens by ~20 mag. Equivalent to factor 10 8 increase in brightness Can outshine an entire galaxy for a few days.

18 High Redshift SN Search Team Harvard-Smithsonian CfA

19 High Redshift SN Search Team Harvard-Smithsonian CfA

20 Type II: Two types of supernova end state of a massive star (as above) M V ~ -17 mag at maximum Type I: occurs in a particular type of binary star system M V ~ -19 mag at maximum Used as Standard Candles : Supernova Cosmology Δm up to ~ 20 mag days

21 Supernova Ejecta Nuclear-processed elements from He to Fe Also new rapid-process heavy elements up to uranium formed during explosion Hence SNe enrich interstellar medium over time Emission lines in optical, X-rays Radio images due to synchrotron radiation from accelerated, charged particles. Examples of SN remnants: Crab nebula (1054 AD) Vela SNR Cygnus loop

22 Crab Nebula

23 Vela Supernova Remnant Optical emission lines X-Rays

24 Cygnus Loop Red = Sulfur: S + Green = Hydrogen: H + Blue = Oxygen: O ++ Jeff Hestsr Arizona State University

25 Pulsars Pulsing radio sources (PSRs) Central core of massive star after a SN event is compressed to nuclear densities (10 17 kg m 3 ) in the form of a neutron star of mass ~ 1.4 M. Star rotates rapidly (conservation of angular momentum) Has very strong magnetic field (conservation of magnetic flux) inclined to rotation axis. Neutron star, radius ~ 15 km! rotation axis magnetic axis

26 Discovery of pulsars Discovered by accident in 1967 by Jocelyn Bell LGM1, LGM2, Soon identified with neutron-star model first proposed theoretically by Fritz Zwicky in 1930s. Cause of pulsed radiation: radiation beamed like a lighthouse near magnetic poles synchrotron radiation from accelerated charged particles Pulse period: Equal to rotation period of neutron star Range from milliseconds to ~ 4 seconds Pulsar slows down as it ages: loss of energy from escaping charged particles emission of radiation decays to radio waves only