Transcript Spectral Classification: The First Step in Quantitative

Spectral Classification:
The First Step in Quantitative Spectral
Analysis
Richard Gray
Appalachian State University
MK Spectral Classification: 1943 – 2013
70 years of contributions to stellar astronomy
• Discovery of the spiral structure of the Galaxy (Morgan,
Sharpless & Osterbrock 1951)
• Link between stellar population & chemical abundances
(Roman 1950, 52, 54)
• Chemical Peculiarities in the A-type stars (Morgan et al.)
• Humphreys-Davidson limit for most luminous stars
• Discovery of the most massive (O2) main-sequence stars
(Walborn et al. 2002)
• Characterization of L- and T-type dwarfs (Kirkpatrick &
Burgasser)
Classification is an Essential Activity of Science
and serves as the
beginning point
for deeper
analysis
The Role of MK Spectral Classification in Spectral Analysis
The MK Spectral type is a Fundamental Datum of Astronomy
if
• The spectral type is obtained solely through comparison with
standards
• Theory & external sources of information are not used in
the determination of the spectral type
If these two principles are followed, the spectral type can serve
as the beginning point for further spectral analysis.
It also serves as a useful “reality check” to an analysis based on
stellar atmosphere theory.
The Two Goals of MK Spectral Classification
1) To locate a star in the context of the broad
population of stars – e.g. its location in the
HR diagram.
2) To identify peculiar and astrophysically
interesting stars
Spectral Analysis begins with the estimation of the
physical parameters: Teff, log(g), [M/H]. How can
Spectral Classification help?
• Spectral classification in conjunction with photometry is
the best way to determine the interstellar reddening
because it does not depend on an extinction model.
• Calibrations of spectral types can give good first
estimates to the star’s Teff, log(g), and even [M/H] and
microturbulent velocity, independent of the reddening.
• The spectral type serves as “ground truth” for checking
the results of further analysis.
Spectral Classification and Spectral Analysis
The second goal – identification of peculiarities and
astrophysically interesting stars is also important for
spectral analysis:
1) Spectral classification can identify stars worthy of
further analysis
2) Knowledge that a star is peculiar in some way is
of vital importance in spectral analysis, as for
some peculiar stars (for instance Ap and Am stars)
the atmospheric structure can deviate strongly
from standard model atmospheres.
How are spectral types determined?
They are determined via direct comparison with standard stars
The spectral region/resolution are of secondary importance,
as spectral classification is no longer confined to the classical
blue-violet region of the spectrum.
Thus, when observing, make certain that you also observe a
few standard stars – always a good idea for any study
OR
Get a colleague with appropriate equipment to observe your
star and a set of standards.
Then – classify your star before you begin your
Spectral Analysis, not as an after thought!
An Example of how to classify a star:
The peculiar F-dwarf, HD 26367
Initial assessment:
Late F (~F8) dwarf with possible chemical peculiarities
We begin by comparing with late F-type dwarf
standards
The Spectral Type of HD 26367
1) Hydrogen lines: F6 – F8
The Spectral Type of HD 26367
1) Hydrogen lines: F6 – F8
2) Metal to Hydrogen ratios: F7
(line ratios)
The Spectral Type of HD 26367
1) Hydrogen lines: F6 – F8
2) Metal to Hydrogen ratios: F7
3) Metallicity independent: ~F7
The Spectral Type of HD 26367
1) Hydrogen lines: F6 – F8
2) Metal to Hydrogen ratios: F7
3) Metallicity independent: ~F7
4) Strength of Metallic-line spectrum: F7
The Spectral Type of HD 26367
1) Hydrogen lines: F6 – F8
2) Metal to Hydrogen ratios: F7
3) Metallicity independent: ~F7
4) Strength of Metallic-line spectrum: F7
5) G-band: F8 – F9
Carbon-rich
Interim conclusion: Assuming luminosity class “V”,
the temperature type of HD 26367 is F7
But we note a carbon peculiarity: G-band too strong
for F7
Luminosity Classification:
Sr II lines 4077 and 4216 are prime luminosity
criteria in late F- and G-type stars
The Sr II lines of HD 26367
luminosity class II
This is inconsistent with the overall appearance of the
spectrum!
Y Sr peculiarity, probably s-process elements
enhanced!
Luminosity Classification
Fall back on secondary luminosity criteria – lines
of Fe II and Ti II
“V” type is valid, so iteration is not necessary
Final spectral type:
F7 V Sr CH+0.4
This classification suggests that HD 26367 belongs to
the group of “Barium Dwarfs”.
These stars have a WD companion, and were
contaminated with s-process elements and carbon
when that WD was an AGB star.
Is HD 26367 a binary star? Yes, both Hipparcos and
radial velocity measurements indicate the presence
of an unseen companion with mass ~ 0.6M
Galex photometry
slight UV excess
WD companion
And HD 26367 is s-process enhanced
Conclusions:
Much of what you will ultimately learn
about your star via spectral analysis can be
anticipated through spectral classification
Spectral classification yields good starting
estimates for the physical parameters of
your star.
Y Spectral Classification is an essential
first step in stellar spectral analysis!