Peculiar A Stars

Heather R. JacobsonA540

13 April 2005

(Sample GHRS spectrum of Lupi, Brandt et al. 1999)

Name Criteria Spectral type*

Teff range (K)

Boö Weak Mg II & weak metals

A0-F0 7500-9500

Am-Fm Weak Ca II, Sc II; enhanced metals

A0-F4 7000-10000

Bp-Ap Enhanced Sr, Cr, Eu &/or Si

B6-F4 7000-16000

HgMn Enhanced Hg II &/or Mn II

B6-A0 10500-16000

He-weak Weak He I B2-B8 14000-20000

He-rich Enhanced He I B2 20000-25000

Chemically Peculiar (CP) Stars:

(Smith 1996)

*they’re not just A stars!

magnetic

CP’s: Overall Properties

(Smith 1996)

CP’s: Overall Properties

(Smith 1996)

CP’s: Overall Properties

(Smith 1996)

CP’s: Overall Properties

(Smith 1996)

Boö Stars- peculiarities first noted in MKK spectral atlas (1943) ( Boötis being the first, of course)

- late B - early F, but are predominantly A-type (only 2%!); low rotational velocities

- CNO and S solar

- Mg, Ca, Ba and Fe-peak underabundant

- ~50 Boö stars known (Gray & Corbally 2002) Faraggiana et al. (2004) report 132 candidates

- low % of Boös indicates the cause of peculiarities has very strict conditions or else is short-lived (Paunzen et al. 2002)

- some exhibit evidence of circumstellar shells

(Gray & Corbally 2002)

Boö spectrum

Boö : Accretion model (Venn & Lambert 1990)

- Dusty circumstellar shell

- Fe-peak, etc. elements with high condensation T’s condence on to dust grains which are blown away by radiation pressure

- CNO & S, with lower condensation T’s, stay in gaseous phase and are accreted by the star

Implies Boös are young stars associated with gas & dust

Observational evidence indicates this is not the case

Are they MS stars with persistent circumstellar disks?

(Gray & Corbally 2002)

BUT: If accretion is the culprit, why are such a small % of stars affected?

Boö: Alternative/Complementary Scenarios

Andrievsky (1997)

- Boös mergers of W Uma type contact binaries

- mass loss during merger could form circumstellar shell

Faraggiana et al. (2004)

- at least a portion of Boös undetected binary systems

- “peculiar” spectra are composites

- up to ~30% of stars Boös studied

Am-Fm Stars

- Metallic line stars

- ID’d by Titus & Morgan (1940); MK class 1943 (Roman, Morgan & Eggen) A0-F4

- some of the coolest CP’s on the MS

- underabundant in Ca & Sc; Fe-peak slightly overabundant; rare earth elements (REE) overabundant

- vsini ≤ 100 km/s

- many are in tight binaries

Sirius is an Am star!

Cf log e(Pb) = -10.15 for the Sun!

(Sadakane 1991)

~1000 x the solar abundance of lead!

Am-Fm Stars: Radiative Diffusion Theory (Michaud 1970)

Chemical differentiation of elements in STABLE atmospheres

- gravity and radiation pressure compete

- some elements go up, some elements go down

Magnetic fields complicate things quite a bit (no surprise…)

“parameter-free” model for HgMn stars after Michaud (figure from Smith 1996); He II convection zone disappears after ~3 Myr

Bp-Ap Stars

- B6-F4 type

- cooler stars (Ap) show Sr, Cr, & Eu enhancements (some show Li overhancements too…)

- hotter stars (Bp) show Si enhancements, Ga too

- some Ap stars are rapid oscillators (roAp); short period, small oscillations

- strong magnetic fields (oblique rotator model)

- alters diffusion process so distribution of elements appears spotty or ring-like

- Bfield strengths similar to WDs’: evolutionary link? (Ferrario & Wickramasinghe 2005)

roAp star HR 3831 (Kochukhov et al. 2004)

roAp star HR 3831 (Kochukhov et al. 2004)

HgMn Stars- B6-A0 spectral type; slow rotators (sharp lines!); perfect for unadulterated diffusion!

- ID’d as having strong Mn lines by Morgan (1931)

- Bidelman identified Hg II as line 3984Å in 1961

- Hg isotopic abundances vary from star to star, with cooler stars containing mostly 204Hg or 202Hg (e.g. Lupi)

- P, Ga & Cu also typically overabundant

- no clear correlation of abundance with physical parameters

- acquisition of UV spectra in the 1990’s has resulted in increased study of these stars

HgMn Stars

Wavelength shifts of different Hg isotopes

(Woolf & Lambert 1999)

HgMn Stars: Lupi

-Leckrone et al. (1999): UV spectrum taken with GHRS

-

Gold!(Brandt et al. 1999)

He-weak Stars- B2-B8 spectral types

- prototype 3 Cen A (Bidelman 1960)

- He I lines weak for ST indicated by photometry and hydrogen lines

- subtypes: P-Ga, Sr-Ti, & Si

- some stars show enhancements of 3He!

(Hartoog & Cowley 1979)

He-rich Stars- B2 spectral type; magnetic; spectra may vary w/time

- example: Ori E (2 symmetric He “caps”)

- He I enhanced, hydrogen lines “normal”

- many found in Orion B

Problems: Current & Future WorkDiffusion.

- many studies have found that radiative diffusion alone cannot sustain some elemental enhancements that are seen (Woolf & Lambert, 1999; Proffitt et al. 1999 [ Lupi]; Kochukhov et al. 2004 [HR 3831])

- hyperfine splitting? Microturbulence?

- do we have magnetic fields right?

- non-LTE affects

- light induced drift (doppler broadening, isotopic splitting) (Aret & Sapar 2002)

Problems: Current & Future WorkThe Usual Suspects in Terra Incognita

- UV spectra of CP’s have resulted in a flurry of studies

- many CP’s show transitions never seen in laboratories

- wavelengths, gf-values, isotopic shifts, hyperfine structure? Much work has been done (e. g. Lupi Pathfinder Project), but much more is needed

- “The Ga Problem” in HgMn stars (Dworetsky et al. 1998)Other observational constraints

- timescales for abundance anomalies -- when do CP’s become CP’s, and for how long?

- many recent studies are simply searches for CP’s in different environmentsWhat’s the deal with binarity?

ReferencesAndrievsky, S. M. 1997 A&A, 321, 838

Aret & Sapar 2002 AN, 323, 21

Brandt et al. 1999 AJ, 117, 1505

Dworetsky et al. 1998 A&A, 333, 665

Faraggiana et al. 2004 A&A, 425, 615

Ferrario & Wickramasinghe 2005 MNRAS, 356, 615

Gray & Corbally 2002 AJ, 124, 989

Hartoog & Cowley 1979 ApJ, 228, 229

Hearnshaw, J. B. 1986 Cambridge University Press: The Analysis of Starlight, pp. 333-351

Kochukhov et al. 2004 A&A 424, 935

Leckrone et al. 1999 AJ, 117, 1454

Paunzen et al. 2002 MNRAS 336, 1030

Proffitt et al. 1999 ApJ, 512, 942

Sadakane, K. 1991 PASP, 103, 355

Smith, K. C. 1996 Ap&SS, 237, 77

Venn & Lambert 1990, ApJ, 363, 234

Woolf & Lambert 1999 ApJ 521, 414

Extras: “Doppler Mapping Inversion” wha??

From Kochukhov et al 2004:

“Stellar surface inhomogeneities such as a nonuniform distribution of temperature and chemical composition lead to characteristic distortions in the profiles of Doppler broaded stellar spectral lines. In the course of stellar rotation, these distortions will move across the line profiles due to changes in visibility and Doppler shifts of individual structures at the stellar surface. The Doppler imaging technique utilizes information contained in rotational modulation of absorption line profiles and reconstructs features at the surfaces of stars by inverting a time series of high-resolution spectra into a map of the stellar surface.”

Extras: Light-induced drift (LID)From Aret & Sapar 2002:

LID occurs in lines with asymmetrical wings. Because of asymmetry, there is asymmetry in the excitation rates of particles with different thermal Doppler shifts.“If the flux in the red wing FR is larger than the flux in the blue wing FB …, there will be more excited downward-moving ions in the atmosphere than upward-moving. The collision cross-section is larger for atomic particles in the excited states than in the ground state….the free paths of particles moving downward sR are shorter than the ones of particles moving upward sB, causing thus an upward flow of particles.”