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 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 • microturbulentvelocity, 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: Spectral classification can identify stars worthy of further analysis 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
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! • 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
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. • Spectral Classification is an essential • first step in stellar spectral analysis!