Peculiar A Stars
Heather R. Jacobson
A540
13 April 2005
(Sample GHRS spectrum of  Lupi, Brandt et al. 1999)
Chemically Peculiar (CP) Stars:
*they’re not just A stars!
Name
Criteria
Spectral
type*
A0-F0
Teff
range (K)
 Boö
Weak Mg II &
weak metals
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
magnetic
7500-9500
(Smith 1996)
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
2%!); low rotational velocities
(only
- 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
 Boö spectrum
(Gray & Corbally 2002)
 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!
~1000 x the solar
abundance of lead!
(Sadakane 1991)
Cf log e(Pb) = -10.15 for the Sun!
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
“parameter-free” model
for HgMn stars after
Michaud (figure from
Smith 1996); He II
convection zone
disappears after ~3 Myr
Magnetic fields complicate things quite a bit (no surprise…)
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
-
(Brandt et al. 1999)
Gold!
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 Work
Diffusion.
- 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 Work
The 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 environments
What’s the deal with binarity?
References
Andrievsky, 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.”