Corv

Corv measures radial velocities and atmospheric parameters for DA-type (hydrogen-atmosphere) white dwarfs observed with BOSS. It was developed by Vedant Chandra and uses the Montreal DA white dwarf model atmospheres.

What it does

Corv fits DA white dwarf models to BOSS optical spectra to determine:

• Radial velocity (v_rad)

• Effective temperature (teff)

• Surface gravity (logg)

The pipeline targets objects in the mwm_wd program that have been classified as DA-type white dwarfs by the Snow White pipeline.

How it works

1. Pre-filtering: Before fitting, each spectrum is checked against three criteria:

   • The source must belong to the mwm_wd program.

   • A Snow White classification must be available.

   • The Snow White classification must indicate a DA-type white dwarf.

   If any of these checks fail, a result is still created but the corresponding flag is set and no fit is performed.

2. Model construction: A DA white dwarf model is built using the Montreal model grid, which provides synthetic spectra parameterized by effective temperature and surface gravity.

3. Radial velocity estimation: An initial radial velocity is determined via cross-correlation on a grid from -1500 to +1500 km/s, followed by quadratic refinement around the peak.

4. Full fit: The pipeline uses lmfit to perform a least-squares fit of the model to continuum-normalized Balmer line profiles in the observed spectrum. The fit iterates over effective temperature to improve convergence.

5. Parallelism: Fitting is run in parallel using a process pool (up to 32 workers by default).

Output fields

Field	Description
v_rad	Radial velocity (km/s)
e_v_rad	Uncertainty on radial velocity (km/s)
teff	Effective temperature (K)
e_teff	Uncertainty on effective temperature (K)
logg	Surface gravity (log cm/s^2)
e_logg	Uncertainty on surface gravity
initial_teff	Initial guess for effective temperature
initial_logg	Initial guess for surface gravity
initial_v_rad	Initial guess for radial velocity
rchi2	Reduced chi-squared of the fit
result_flags	Bitfield summarizing result quality

Flags

Flag	Bit	Meaning
flag_not_mwm_wd	2^5	Object is not in the mwm_wd program
flag_no_wd_classification	2^6	No Snow White classification is available
flag_not_da_type	2^7	Object is not classified as DA-type by Snow White

When any of these flags are set, no fit was performed and the stellar parameter and radial velocity fields will be null.

Key caveats

• Corv only fits DA-type white dwarfs. Other white dwarf subtypes (DB, DC, DQ, DZ, etc.) are not supported and will be flagged.

• Results depend on the Snow White classification being correct. Misclassified objects will either be skipped or produce unreliable fits.

• The Montreal DA model grid has finite coverage in temperature and gravity. Results near the grid edges should be treated with caution.

• The pipeline operates on BOSS visit spectra, not coadded spectra.