The Stellar Zoo The Stellar Zoo Weird stars across the HR diagram What causes stellar abundance anomalies?
Feb 06, 2016
The Stellar ZooThe Stellar Zoo
Weird stars across the HR diagram
What causes stellar abundance anomalies?
The Upper Upper Main The Upper Upper Main SequenceSequence
• 100 (or so) solar masses, T~20,000 – 50,000 K• Luminosities of 106 LSun
• Generally cluster in groups (Trapezium, galactic center, eta Carinae, LMC’s R136 cluster)
(Some of) The Brightest Stars in the Galaxy
Star mV MV Mbol Sp. T. Dist.Pistol Star … … -11.8 7 kpc
HD 93129A 7.0 -7.0 -12 O3If 3.4 kpc
Eta Carina 6.2 -10 -11.9 B0 0 2.5 kpc
Cyg OB2#12
11.5 -10 -10.9 B5 Ia+e 1.7 kpc
Zeta-1 Sco 4.7 -8.7 -10.8 B1.5 Ia+ 1.9 kpc
Wolf-Rayet StarsWolf-Rayet Stars• Luminous, hot supergiants• Spectra with emission lines• Little or no hydrogen
• 105-106 Lsun
• Maybe 1000 in the Milky Way
• Losing mass at high rates, 10-4 to 10-5 Msun per year
• T from 50,000 to 100,000 K
•WN stars (nitrogen rich)•Some hydrogen (1/3 to 1/10 HE)•No carbon or oxygen
WC stars (carbon rich)NO hydrogenC/He = 100 x solar or moreAlso high oxygen
•Outer hydrogen envelopes stripped by mass loss•WN stars show results of the CNO cycle•WC stars show results of helium burning•Do WN stars turn into WC stars?
Types of Massive StarsTypes of Massive Stars
• Luminous Blue Variables (LBVs)– Large variations in brightness (9-10 magnitudes)– Mass loss rates ~10-3 Msun per year, transient rates of 10-1
Msun per year– Episodes of extreme mass loss with century-length
periods of “quiescence”– Stars’ brightness relatively constant but circumstellar
material absorbs and blocks starlight– UV absorbed and reradiated in the optical may make the
star look brighter– Or dimmer if light reradiated in the IR– Hubble-Sandage variables are also LBVs, more frequent
events– Possibly double stars?– Radiation pressure driven mass loss?– Near Eddington Limit?
Venn 1995: Fig 11Venn 1995: Fig 11
[C/N] ratios in A supergiants vs. effective temperature.The dashed line is the mean [N/C] ratio for B stars; the dotted line is the first dredge-up prediction for a 10solar mass star.
Chemically Peculiar Stars of Chemically Peculiar Stars of the Upper Main Sequencethe Upper Main Sequence
• Ap stars– SrCrEu stars– Silicon Stars– Magnetic
fields– Oblique
rotators– Slow rotators
• Am-Fm stars– Ca, Sc deficient– Fe group,
heavies enhanced
– diffusion• HgMn stars• The Boo stars• Binaries?
Solar Type StarsSolar Type Stars
• Pulsators– The delta Scuti stars– SX Phe stars
• Binaries– FK Comae Stars– RS CVn stars– W UMa stars– Blue Stragglers
Boesgaard & Tripicco 1986: Boesgaard & Tripicco 1986: Fig 2Fig 2
The famous lithium dip!
The Lower Main SequenceThe Lower Main Sequence• UV Ceti Stars
– M dwarf flare stars– About half of M dwarfs are flare stars (and a few K
dwarfs, too) – A flare star brightens by a few tenths up to a magnitude
in V (more in the UV) in a few seconds, returning to its normal luminosity within a few hours
– Flare temperatures may be a million degrees or more– Some are spotted (BY Dra variables)– Emission line spectra, chromospheres and coronae; x-ray
sources– Younger=more active– Activity related to magnetic fields (dynamos)– But, even stars later than M3 (fully convective) are active
– where does the magnetic field come from in a fully convective star?
– These fully convective stars have higher rotation rates (no magnetic braking?)
On to the Giant Branch…On to the Giant Branch…
• 1st dredge-up• LF Bump• Proton-capture
reactions
• Carbon Isotopes• Lithium
Gilliland et al 1998 (47 Tuc)
[[/Fe] Variations /Fe] Variations with Metallicity for Globular with Metallicity for Globular Clusters and Field StarsClusters and Field Stars
–Stellar data from Fulbright 2000
-0.3
0
0.3
0.6
-2.5 -2 -1.5 -1 -0.5[Fe/H]
[Ca/Fe]
CN-weak giants in M22 have lower metallicity than CN-strong giants (Brown and Wallerstein 1992)
[[/Fe] Variations within Clusters/Fe] Variations within Clusters
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
Mg Si Ca Fe
CN-weak
CN-strong
[m/H]
The [Ba/Eu] RatioThe [Ba/Eu] Ratio
Stellar Data from Fulbright 2000 and Burris et al. 2000
-1
-0.5
0
0.5
-2.5 -2 -1.5 -1 -0.5
[Fe/H]
[Ba/Eu]
Abundance Changes on the Giant BranchAbundance Changes on the Giant Branch
• Bellman et al. show a steady decline in the carbon abundance as M92 giants evolve up the giant branch
Oxygen and SodiumOxygen and Sodium
• Ivans et al. (1999) show a remarkable O vs. Na anti-correlation in several clusters
CH
Nature AND Nurture in M71Nature AND Nurture in M71
CN
Cohen 1999 Penny et al. 1992
CH
CN
GiantsMain Sequence Stars
Explaining Abundance Changes:Explaining Abundance Changes:
•Proton-capture nucleosynthesis on the giant branch
22Na
NeNa Cycle
23Na
20Ne
21Na21Ne
22Ne
24Mg
27Al
26Mg
26Al
25Mg25Al
MgAl Cycle
28Si
NGC 6752’s NGC 6752’s Main SequenceMain Sequence
• VLT data show sodium and oxygen anti-correlation in unevolved stars
Briley et al. 1996: Fig 1Briley et al. 1996: Fig 1
The 2The 2ndnd Parameter Parameter ProblemProblem
• Two clusters of similar age and metal content show different horizontal branch morphology
Pilachowski et al. 1993: Fig 8Pilachowski et al. 1993: Fig 8
Carbon Isotope RatioCarbon Isotope Ratio
Charbonnel 1995: Fig 1Charbonnel 1995: Fig 1
Real Red GiantsReal Red Giants• Miras (long period variables)
– Periods of a few x 100 to 1000 days– Amplitudes of several magnitudes in V (less in K near flux
maximum)– Periods variable– “diameter” depends greatly on wavelength– Optical max precedes IR max by up to 2 months– Fundamental or first overtone oscillators– Stars not round – image of Mira– Pulsations produce shock waves, heating photosphere,
emission lines– Mass loss rates ~ 10-7 Msun per year, 10-20 km/sec– Dust, gas cocoons (IRC +10 216) some 10,000 AU in
diameter• Semi-regular and irregular variables (SRa, SRb, SRc)
– Smaller amplitudes– Less regular periods, or no periods
Amplitude of Mira Light Amplitude of Mira Light CurveCurve
More Red GiantsMore Red Giants• Normal red giants are oxygen rich – TiO dominates the spectrum• When carbon dominates, we get carbon stars (old R and N
spectral types)• Instead of TiO: CN, CH, C2, CO, CO2• Also s-process elements enhanced (technicium)• Double-shell AGB stars
Peery 1971
Weirder Red GiantsWeirder Red Giants
• S, SC, CS stars– C/O near unity – drives molecular equilibrium to weird
oxides• Ba II stars
– G, K giants– Carbon rich– S-process elements enhanced– No technicium– All binaries!
• R stars are warm carbon stars – origin still a mystery– Carbon rich K giants– No s-process enhancements– NOT binaries– Not luminous for AGB double-shell burning
• RV Tauri Stars
Smith & Smith & Lambert Lambert 1989: Fig 1989: Fig 11Red giants in theMagellanic Clouds
Mass Transfer BinariesMass Transfer Binaries
The more massive star in a binary evolves to the AGB, becomes a peculiar red giant, and dumps its envelope onto the lower mass companion
• Ba II stars (strong, mild, dwarf)• CH stars (Pop II giant and subgiant)• Dwarf carbon stars• Nitrogen-rich halo dwarfs• Li-depleted Pop II turn-off stars
McCarthy & Nemec 1997: Fig McCarthy & Nemec 1997: Fig 1111
Analysis of the anomalous Cepheid V19 in the globular cluster NGC 5466
After the AGBAfter the AGB
• Superwind at the end of the AGB phase strips most of the remaining hydrogen envelope
• Degenerate carbon-oxygen core, He- and H-burning shells, thin H layer, shrouded in dust from superwind (proto-planetary nebula)
• Mass loss rate decreases but wind speed increases• Hydrogen layer thins further from mass loss and He burning shell• Star evolves at constant luminosity (~104LSun), shrinking and
heating up, until nuclear burning ceases• Masses between 0.55 and 1+ solar masses (more massive are
brighter)• Outflowing winds seen in “P Cygni” profiles• Hydrogen abundance low, carbon abundance high (WC stars)• If the stars reach T>25,000 before the gas/dust shell from the
superwind dissipates, it will light up a planetary nebulae• Temperatures from 25,000 K on up (to 300,000 K or even higher)• Zanstra temperature - Measure brightness of star compared to
brightness of nebula in optical hydrogen emission lines to estimate the uv/optical flux ratio to get temperature