RESEARCH EXPERIENCE

October 2019 - May 2021

JUNIOR INVESTIGATOR

Gehan et al. submitted to Astronomische Nachrichten

October 2018 - September 2019

RESEARCH AND TEACHING ASSISTANT

Observatoire de Paris, Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA), France

Attachée Temporaire d'Enseignement et de Recherche (ATER position): research position with 96h of teaching duties.

RESEARCH

Automated approach to measure stellar inclinations: validation through large-scale measurements on the RGB

- Development of a method based on the ratio between power spectral densities of modes with different azimuthal orders to derive automated asteroseismic measurements of the inclination angle between the stellar rotation axis and the line-of-sight.

- Application on ~1200 stars on the RGB from the Kepler Input Catalog.

The distribution of raw measurements strongly deviates from the expected isotropic distribution, due to the fact that extreme inclinations are inaccessible to measurements since they require an infinte signal-to-noise ratio. Only upper (i.e. lower) estimates can be derived for low (i.e high) inclinations, resulting in a highly biased distribution.

- Results: taking into account individual uncertainties through probability density functions allows us to recover the expected isotropic distribution, validating the method. This result highlights that we have to correct the observational bias in order to retrieve the underlying inclination distribution of a given stellar sample.

- Skills: Kolmogorov-Smirnov & null-hypothesis statistical tests.

Gehan et al. (2021)

October 2015 - September 2018

PHD CANDIDATE

Observatoire de Paris, Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA),

Complementary missions:

- 2 years of teaching duties (128h)

- 1 year of scientific outreach (128h)

RESEARCH

Core rotation braking on the red giant branch for various mass ranges

- Development and application of a method allowing largely automated measurements of the mean core rotation of stars on the RGB.

- Mean core rotation measurements for ~900 RGB stars from the Kepler Input Catalog with masses between 1 and 2.5 solar masses.

- Results: while previous studies emphasized that the mean core rotation is slightly slowing down along the RGB, our results instead suggest that the mean core rotation is constant along RGB, with values independent of the mass. This striking result may turn out to be crucial to understand the physical mechanism(s) transporting angular momentum inside red giants.

Implications on the efficiency of the angular momentum extraction from the core through modelling

- Quantifying the angular momentum extraction rate from each layer of the contracting core, along the RGB for different stellar masses, in order to reproduce the constant core rotation measured by Gehan et al. (2018).

- Computation of profiles of the internal moment of inertia at different timesteps on the RGB and for different masses using stellar models derived with the evolutionary code Modules for Experiments in Stellar Astrophysics (MESA).

- Models indicate that the angular momentum extraction rate from the inert helium core:

1. increases with stellar mass;

2. increases over time for low-mass star but decreases over time for intermediate-mass stars.

- Skills: Hough transform, chi-square minimization.

Gehan et al. (2018)

Centro de Astrofísica da Universidade do Porto (CAUP), Portugal

Full-time research position.

RESEARCH

FRA – A new Fast, Robust and Automated pipeline for the detection and measurement of solar-like oscillations in time-series photometry of red-giant stars

- Automated measurement of the frequency of maximum oscillation power based on a local fit of the envelope of oscillations and of the background contribution to the smoothed oscillation spectrum; quantification of the reliability of the detection based on the likelihood-ratio test.

- Automated measurement of the large separation using the Envelope Autocorrelation Function (EACF) method; quantification of the reliability of the detection based on the rejection of the null-hypothesis.

- Application on ~250 artificial power spectra representative of TESS red giants as well as ~2300 TESS and ~1700 Kepler red giants.

- Results: the obtained consistency rate compared with existing measurements is above 99% for Kepler red giants and above 97% for TESS red giants. Using the frequency of maximum oscillation power as an input parameter to guide the search for the large separation through the (EACF) method significantly improves the consistency of the measured large separation in the case of TESS stars. We can expect to get consistent measurements while minimizing both the false positive measurements and the non-detections for stars with a minimum of 110 days (four observed TESS sectors) and a maximum G magnitude of 9.5.

- Skills: Fourier transform, spectrum filtering & smoothing, likelihood-ratio statistical test.