The Stellar Zoo

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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

-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

Mg Si Ca Fe

CN-weak

CN-strong

The [Ba/Eu] RatioThe [Ba/Eu] Ratio

Stellar Data from Fulbright 2000 and Burris et al. 2000

-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

Nature AND Nurture in M71Nature AND Nurture in M71

Cohen 1999 Penny et al. 1992

GiantsMain Sequence Stars

Explaining Abundance Changes:Explaining Abundance Changes:

•Proton-capture nucleosynthesis on the giant branch

NeNa Cycle

21Na21Ne

25Mg25Al

MgAl Cycle

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

The Stellar Zoo

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