Spectral Classification:The First Step in Quantitative Spectral Analysis

Richard GrayAppalachian State University

MK Spectral Classification: 1943 – 201370 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 Scienceand serves as thebeginning pointfor deeperanalysis

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 serveas 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 thephysical parameters: Teff, log(g), [M/H]. How canSpectral 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 andastrophysically interesting stars is also important forspectral analysis:

1) Spectral classification can identify stars worthy of further analysis2) 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 classicalblue-violet region of the spectrum.

Thus, when observing, make certain that you also observe afew standard stars – always a good idea for any study

OR

Get a colleague with appropriate equipment to observe yourstar and a set of standards.

Then – classify your star before you begin yourSpectral 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 – linesof 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 tothe group of “Barium Dwarfs”.

These stars have a WD companion, and werecontaminated with s-process elements and carbonwhen that WD was an AGB star.

Is HD 26367 a binary star? Yes, both Hipparcos andradial velocity measurements indicate the presenceof 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 learnabout your star via spectral analysis can be anticipated through spectral classification

Spectral classification yields good startingestimates for the physical parameters ofyour star.

Y Spectral Classification is an essentialfirst step in stellar spectral analysis!