Variability study of the high-resolution spectra of an O-star

Guilherme D. C. TeixeiraSupervisors: M. S. Nanda Kumar, M. J. P. F. G. Monteiro

Stellar Structure and Evolution

● Described and predicted by structure equations

∂m∂r

=4 π r2ρ

∂ P∂ r

=−Gm

r2ρ

∂ L∂ r

=4π r2ρε

∂T∂r rad

=−34ac

κρ

T 3

L

4 π r2

∂T∂r ad

=−(1−1γ )

μmhκ

Gm

r2

Mass conservation

Hydrostatic Equilibrium

Energy Equation

Radiative Transport

Adiabatic Convection

H-R diagram

Figure - The Hertzsprung-Russel diagram. Credits: ESO

Internal structures

● Internal structure of stars is dependent on mass

Figure - Heat transfer on stars. Credit: Sun.org - www.sun.org

O-stars

● Massive stars are rare(~1% of all stars).

● Short-lived (~106yrs).● Responsible for

synthesis of heavyelements via supernovae.

Figure - Number of stars in Eagle Nebula by mass bin. From Andrea Stolte - private communication.

Where to look for O-stars?

● Nearby star forming/HII regions

● The moleculargas is being ionizedby the embeddedO stars

● Orion, Monoceros,M8, Trifid are someof the regions wewill study

Figure - Color composite made by WISE, shows Orion. Bright red arc on bottom right is one of our targets, Sigma Orionis. Credit: NASA.

Getting R,M

● Photometric methods: – Mass-Luminosity relation (M)

– Interferometry (R)

– Evolutionary tracks (M, R)

● Spectroscopy:– Effective temperature (R)

– Logg (M)

– Spectral type (M, R)

● Asteroseismology– Gravity modes and pressure modes (R,M)

Asteroseismology

● Observes oscillations and variability on stellar flux and spectra

● Compares with models of stellar pulsations

● Different oscillation modes probe stellar interior at different depths

● Measures precise stellar parameters Figure - Principle of asteroseismic modelling.

From Aerts (2015).

p-modes and g-modes

● p-modes, or pressure modes, are present in convection dominated envelopes. Do not propagate in radiative zones.

● g-modes, or gravity modes, show up in radiative zones. Do not propagate in convective zones.

● mixed modes are combination of p- and g- modes

Difficulties of massive star asteroseismology

● Different internal structures from low- and intermediate-mass stars

● Fast rotation

● Mass loss from stellar winds

● Stronger magnetic fields

● Smaller number of sharp absorption lines

Macroturbulence

Credit David F. Gray

● Had-hoc velocity field that explains the shape of the wings of a spectral line (Struve 1952; Conti & Ebbets 1977; Howarth et al. 1997).

● No physical explanation for its presence

High-resolution spectra in O-stars

● Spectral lines are highly-broadened

● Show presence of macroturbulence

● Macroturbulence shown not to be product of large scale turbulence motions (Simón-Díaz et al. 2010).

Broadening and the Pulsational Hypothesis

● macroturbulence is a collective pulsational velocity broadening due to g-modes (Aerts et al. 2009; Simón-Díaz et al. 2010)

● Pulsational components have been confirmed on B-dwarfs (Aerts et al. 2009, Waelkens et al. 1998).

Figure - Noiseless pulsationally and rotationally broadened profiles (thin lines) compared with only rotational broadening (dashed). From Aerts et al. (2009).

Line variability in O-stars● Explored in Simón-Díaz (2015).● Observational strategy based on long-period variability.● Observed multi-periodic variability from high-order g-modes● Found correlation between the skewness of profile and

macroturbulent velocity

Figure - Line-profile variability of B star on the IACOB sample. From Simón-Díaz (2015).

Our targetSpectral Type ~ O9

Luminosity ~ 40000 Lsun

Mass ~ 20 Msun

Teff ~ 35000 K

age ~ 300 000 yrs

v sini ~ 140 km/s

radial velocity -29.45 km/s

visual magnitude ~ 4 mag

log g ~ 4.2 dex

Building a Linelist

● Stellar atmospheres depend on: age, mass, energy, chemical composition, and momentum of a given star

● One of primary tasks was to build a line list appropriate to young O-stars

Figure - Spectra for different spectral types. From Jacoby et al. (1984).

Studied lines

● Spectral lines based on careful study of the available spectra

● Selected lines with line depth >30% in synthetic spectra of O star

Observations● Used the PRL Advanced

Radial-velocity All-sky Search spectrograph at the 1.2m telescope at Mt. Abu, India

● Has a wavelength coverage of 3800-6900 Å

● Spectral Resolution of R~67000

Figure – Echelle spectra image.Credit: “PARAS First light”

Summary of observations

CCF

● Cross-correlation function (CCF) correlates the spectrum with a binary mask

Figure - Construction of CCF with template binary mask. From Melo (2001).

Temporal Variance Spectrum analysis

TVSi=1

N−1∑i=1

N

d ij2

TVS of He 4713 from Martins et al. 2010 TVS of He 4920 from Martins et al. 2010

CCF variability

CCF of 2015-01-17 for each pair of two observations. The black line represents the median of all days

The difference between the median CCF and the observation at each interval of 3 km/s

CCF variability

CCF of 2015-01-18 for each pair of two observations. The black line represents the median of all days

The difference between the median CCF and the observation at each interval of 3 km/s

CCF variability

CCF of 2015-01-19 for each pair of two observations. The black line represents the median of all days

The difference between the median CCF and the observation at each interval of 3 km/s

H-alpha variability (daily)

Variability of the H-alpha line in Jan of 2015. Each line represents a consecutive day and the black line represents the median of the month.

TVS of the H-alpha line in Jan of 2015. The dotted, dashed and full black lines represent, respectively the 1-sigma, 2-sigma and 3-sigma of the wings.

He-5875 variability (daily)

Variability of the He-5875 line in Jan of 2015. Each line represents a consecutive day and the black line represents the median of the month.

TVS of the He-5875 line in Jan of 2015. The dotted, dashed and full black lines represent, respectively the 1-sigma, 2-sigma and 3-sigma of the wings.

H-alpha vs He 5875

EW of H-alpha vs Ew of He 5875

Possible explanations for variability

● Short-lived prominences (Sudnik et al. 2016)

● Wind variability (Martins et al. 2015)

● Non-radial Pulsations (Fullerton et al. 1996)

Future work

● Implementing analysis based on the bisector method

● Further study the correlation between the variations of different lines

● Model and minimize the effect of winds

Figure - Bisector method. From Dravins (2008).

Summary

● We have found spectral variability in an O-star

● Variability is present in the timescale of days and hours

● We will further improve our study by modeling and minimizing other effects such as winds