parvati 1.0.5

A package to compute and analyse stellar line profiles

PARVATI

PARVATI (Profiles Analysis and Radial Velocities using Astronomical Tools for Investigation) is a Python package for the analysis of astronomical spectra.

Introduction

PARVATI contains several useful functions that allow to create mean line profiles or extract single spectroscopic lines from ASCII or FITS spectra. The line profiles may then be used to compute the radial velocities, the projected rotational velocities, the equivalent widths, the line moments, and other additional scientific information. This README file will briefly cover all the available PARVATI functions, but a more detailed description of the functions and their input parameters may be obtained with:

import parvati as pa
help(pa)
help(pa.read_spectrum)
help(pa.FUNCTION)

PARVATI may be either downloaded from GitHub or installed using pip:

python -m pip install parvati

Preparation of the spectra

The input spectra may have different formats: either ASCII files with wavelength, flux and additional information (e.g., SNR, errors, or echelle order number), standard monodimensional FITS files or FITS tables. The spectra may be read with the read_spectrum function. Before extracting the line profiles they must be normalised, e.g., using the norm_spectrum function.

Read the spectra

The function read_spectrum will require as input the filename of the spectrum, and accordingly to the other options given it will read:

• a monodimensional FITS file with the flux as the hdu[0].data and the wavelength in the hdu[0].header (CRVAL1, CDELT1, NAXIS1)
• a FITS table with all the data in hdu[1].data. By default, the wavelength will be read in the first field and the flux in the second field, but the number of the field may be specified. If there are any additional data as SNR and/or echelle order number and/or normalised flux and/or absolute errors, they may be specified here. If given, the S/N supersedes the errors, otherwise the errors will be transformed in S/N (S/N=flux/errors). IMPORTANT: the numbers of the fields start with 1, not 0
• an ASCII files with at least two columns (wavelength and flux), but additional columns with SNR and/or echelle order number and/or normalised flux and/or absolute errors may be specified here. If given, the S/N supersedes the errors, otherwise the errors will be transformed in S/N (S/N=flux/errors). IMPORTANT: the numbers of the columns start with 1, not 0

The wavelength is assumed to be in Angstroms, if otherwise then it must be specified using the unit parameter.

Normalise the spectra

If the spectra are not normalised, it is possible to use the function norm_spectrum to do so. The function requires at least the wavelength and flux as input. It is possible to set the degree of the polynomial and a number of subsets to be normalised independently.

If the refine option is set to Trueand either echelle orders or the subsets are given, then the normalisation may be refined by ensuring a smooth variation of the polynomial coefficients along the orders.

Creation of the profiles

Once in possession of normalised spectra, the line profiles may be extracted or created in several different ways, as detailed below.

Single line extraction

The function extract_line requires the spectrum in the format given by read_spectrum or norm_spectrum, the laboratory wavelength of the desired line, the radial velocity range and step of the extraction window. The extracted line will be interpolated on a Doppler velocity range, to help with the subsequent analysis.

Mean line profile: LSD

The function compute_lsd compute the mean line profile by performing a Least-Squares Deconvolution (LSD) of the spectrum in the format given by read_spectrum or norm_spectrum with a mask that may be either an ASCII mask (VALD stellar mask, simple 2-columns file, normalised spectrum/model) or a FITS file (mask or normalised spectrum/model). Several options may be passed to optimised the profile extraction.

The LSD computation has been adapted from Donati J.-F., et al., 1997, MNRAS 291, 658 and Kochukhov O., et al., 2010, A&A 524, 5.

IMPORTANT: to save computational time, the spectra should be split in subsets, and LSD profiles are computed for each subset independently and then averaged. This is naturally done using the echelle orders/subsets option in norm_spectrum: compute_lsd will automatically find the separations between the orders where the wavelength gap is larger than the average wavelength step or if there is overlapping.

Mean line profile: CCF

The function compute_ccf compute the mean line profile by performing a Cross-Correlation of the spectrum in the format given by read_spectrum or norm_spectrum with a mask that may be either an ASCII mask (VALD stellar mask, simple 2-columns file, normalised spectrum/model) or a FITS file (mask or normalised spectrum/model). Several options may be passed to optimise the profile extraction.

IMPORTANT: to save computational time, the spectra should be split in subsets, and CCF profiles are computed for each subset independently and then averaged. This is naturally done using the echelle orders/subsets option in norm_spectrum: compute_ccf will automatically find the separations between the orders where the wavelength gap is larger than the average wavelength step or if there is overlapping.

Analysis of the profiles

Once the profiles have been obtained, PARVATI allows to perform several useful operations on the data, to better analyse them.

Normalise the profiles

First of all, even if the profiles should already be normalised, any slight deviation from a perfect normalisation of the spectra will impact on the normalisation of the profiles. It is better to re-normalised them using the norm_profile function. This function requires as input not the name of a single file, but an ASCII file with a list of names: this will allow to compute also an average mean line profile and the standard deviation of the profiles from this average, to better see where possible line profile variations are located.

Fit the profiles

Once normalised, the profiles may be fitted using the fit_profile function. It is possible to choose one or more different fitting functions: Gaussian, Lorentzian, Voigt or rotational profile. All the fitting functions yield a Radial Velocity (RV) estimation, the Equivalent Width (EW) and other information depending on the function (e.g. the vsini from the rotational function). The rotational function is taken from Gray, D. F. 2008, The Observation and Analysis of Stellar Photosphere.

Compute the line moments

The first four line moments may be computed from the (normalised) profiles using the moments function. The moments are:

• m0: EW
• m1: RV
• m2: sigma, from which also the Full-Width-at-Half-Maximum (FWHM) is derived
• m3: from which the skewness is derived
• m4: from which the kurtosis is derived.

The definition for the moments is taken from Briquet M. & Aerts C., 2003, A&A 398, 687, and the estimations of the errors from Teague R., 2019, Res. Notes AAS 3, 74.

Compute the line's bisector

The functionbisector computes both the bisector of the line and the bisector's span. The error computation and the definition of the bisector's span are taken from Baştürk Ö., et al., 2011, A&A 535, 17.

Compute the Fourier Transform of the line

The function fourier first symmetrises the line and then performs the Fourier Transform (FT) of the symmetrised line. The symmetrisation process yields another estimate of the RV (but without any associtated error), while the positions of the first 3 zeroes of the FT give information on the vsini and the differential rotation IF the rotational broadening is the dominant line broadening effect. The vsini is derived using the empirical formula from Dravins, D., Lindegren, L., & Torkelsson, U. 1990, A&A, 237, 137.

Test files

A couple of simple scripts are given in the tests directory along with two high-resolution spectra and two VALD stellar masks to guide in computing/extracting the line profiles and then analysing them.

Graphical interface: SHIVA

The graphical interface of PARVATI is SHIVA (Simple and Helpful Interface for Variability Analysis). SHIVA may be downloaded from https://github.com/mrainer74/shiva