MDwarfType

MDwarfType assigns spectral types to M dwarf stars by matching observed BOSS spectra against a library of spectral templates.

What it does

MDwarfType determines:

  • Spectral type (e.g., “M0.0”, “M3.5”, “K7.0”)

  • Numerical sub-type

  • A goodness-of-fit statistic

How it works

  1. Template preparation: A library of M dwarf spectral templates (covering types K5 through late M) is loaded and resampled onto the standard BOSS wavelength grid. Each template is rectified by fitting and dividing out a quadratic continuum, restricted to the wavelength range 5000–8800 Angstroms.

  2. Continuum normalization: The observed spectrum is normalized by its mean flux in a narrow window around 7500 Angstroms (7495–7505 A), then rectified with the same quadratic procedure used for the templates.

  3. Template matching: The chi-squared statistic is computed between the rectified observed spectrum and each rectified template, weighted by the inverse variance. The template with the lowest chi-squared is selected as the best match.

  4. Result: The spectral type and sub-type from the best-matching template are reported, along with the reduced chi-squared of the match.

  5. Parallelism: Spectra are processed in batches using a process pool.

Output fields

Field

Description

spectral_type

Best-matching spectral type string (e.g., “M3.5”)

sub_type

Numerical sub-type (e.g., 3.5)

rchi2

Reduced chi-squared of the best template match

continuum

Mean continuum level (flux in the 7500 A normalization window)

result_flags

Bitfield summarizing result quality

Flags

Flag

Bit

Meaning

flag_suspicious

2^0

Best-matching spectral type is K5.0, which is suspicious for an M dwarf target

flag_exception

2^1

A runtime exception occurred during processing

Composite flags

  • flag_bad: Set if any flag bit is nonzero (i.e., result_flags > 0).

Key caveats

  • MDwarfType is a template-matching classifier, not a physical model fit. The spectral type reflects the best match from a discrete set of templates.

  • If the best match is K5.0, the flag_suspicious flag is raised because K5 is at the boundary of the M dwarf regime and may indicate a misclassified target.

  • The template library and wavelength coverage (5000–8800 A) are optimized for BOSS optical spectra. The method relies on features in the red optical.

  • The quadratic rectification removes the broad-band spectral shape, so the classification is driven by molecular band strengths and other spectral features rather than the overall continuum slope.