University of Groningen

Joint optical and near-infrared spectroscopic studies of stars with X-shooterGonneau, Anais

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Citation for published version (APA):Gonneau, A. (2015). Joint optical and near-infrared spectroscopic studies of stars with X-shooter: Aninsight into carbon stars. University of Groningen.

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English summary? ? ?

The leitmotiv for this thesis has been the understanding of the formation and theevolution of galaxies. To contribute pieces of the puzzle, we tackled this issuefrom a different point of view: we looked at all the stars that form a galaxy. Tobe sure, a galaxy is more than just billions of gravitationally-bound stars. It alsoconsists of stellar remnants, interstellar gas and dust, and dark matter. The colorof a galaxy is mainly due to the type of stars that compose the galaxy. Therefore,by comparing the spectrum of a galaxy with that of a star, it is possible to traceback the type of stars that prevalent within a galaxy. Recipes are used to modelthe spectral energy distribution (SED) of galaxies. We call this approach stellarpopulation synthesis (SPS). The key ingredient, according to my – biased – pointof view, is a set of libraries of stellar spectra. These libraries, either theoretical orempirical, are used to convert the outputs of stellar evolution calculations, suchas surface gravity, effective temperature, into observable SEDs.

In this thesis, we chose to create a new empirical library. Besides the large num-ber of existing libraries, empirical stellar libraries need improvements on the follo-wing points: better calibrations, more stars, higher spectral resolution and broaderwavelength coverage. The technical progress made over the last decades leads tothe emergence of new instruments, including the X-shooter spectrograph. Thisspectrograph, property of ESO (European Southern Observatory) and mountedat the Very Large Telescope (VLT) in Chile, offers the unique capability to pro-duce in one shot a full spectrum from the ultra-violet wavelength range to thenear-infrared (from 300 to 2480 nm), at a mean resolving power of 10 000.

During six semesters, we observed with X-shooter more than 700 stars from avariety of environments (from the nearby disc of the Milky Way to the MagellanicClouds), with a variety of chemical compositions. All these stars are part of theX-shooter Spectral Library (XSL, P.I. S. C. Trager). Figure 5 shows an overviewof our sample in a Hertzsprung-Russell diagram. The symbols represent the twophases of our survey: a Pilot survey (two first semesters) and the Large Programme(four semesters).

A large part of my thesis was devoted to the data reduction, in other wordstransfor-ming the raw data into final one-dimensional spectra, ready for scienceapplication. To do so, we combined the tools provided by ESO with proceduresof our own. One aspect of the data reduction process is shown in Chapter 4. Thisconcerns the computation of the instrumental response curve, prior to the fluxcalibration. X-shooter is a ground-based instrument, consequently, the spectra areaffected by the Earth’s atmosphere (also called telluric features). This is mainlyseen in the near-infrared wavelength range, and to a lesser extent in the visible.Figure 6 shows an example of a near-infrared spectrum of a flux standard star,and the parts of the spectrum affected by the telluric absorption (in green) and

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the instrumental features (in blue). Because of the intimate relation between thesetwo effects, a procedure was developed to deal with both aspects at the same time.

31The Messenger 158 – December 2014

including bias and/or dark correction, flat-fielding, geometric correction, wavelength calibration and (when necessary) sky subtraction. A number of issues were dis-covered during the pipeline reduction process, which required further process-ing steps. We give details about two of the most significant of these steps here; more details can be found in Chen et al. (2014).

Telluric correctionGround-based observations are always subject to contamination from the Earth’s atmosphere. In the visible and near- infrared portions of the spectrum, water vapour, molecular oxygen, carbon dioxide and methane generate strong absorption features that originate in the Earth’s atmosphere and are referred to as telluric features. Corrections for telluric con tamination therefore are important for the XSL spectra in the VIS and NIR arms.

We used “telluric standard star” obser-vations taken as part of the standard X-shooter calibration plan directly after each of our science observations as a basis for telluric correction of our optical data. These stars are typically B2–A0 dwarfs, whose intrinsic spectra contain only H and He absorption lines on top of nearly pure blackbody spectra. We determined the telluric absorption spec-trum at the time of observation of each of these stars by dividing the ob served spectrum by a model of the star’s intrin-sic spectrum. We found that the telluric absorption lines changed strength on timescales shorter than the “long” expo-sure time (> 90 seconds) of faint XSL stars and the total overhead time of ~ 900 seconds, resulting in an imperfect telluric correction. In addition, small changes in spectral resolution and wave-length zero-point occurred even between successive observations. To optimise the telluric correction, we therefore built a library of telluric spectra, in which 152 telluric standard stars were carefully wavelength-calibrated and had their intrinsic spectra modelled and removed.

We developed a principal-component-analysis- (PCA) based method that can quickly and precisely perform telluric corrections for warm stars in XSL. Figure 3 shows the first six principal com ponents from the bottom to top. We

the narrow-slit “nodding” observations and good flux calibration from the wide-slit “staring” observations. The pro-gramme was designed to fill any gaps in the observing queue, and therefore the observations were typically taken during poor seeing and thick cloud conditions; this however did not affect the relative flux calibration precision (see below).

Data reduction

The data reduction of the near-ultraviolet and optical spectra from the pilot survey was performed with the public release of the X-Shooter pipeline version 1.5.0, fol-lowing the standard steps described in the X-shooter pipeline manual1 up to the production of two-dimensional spectra,

Figure 1. (Above) The Hertzsprung–Russell diagram of XSL stars (surface gravity log g as a function of effective temperature Teff), colour-coded by metallic-ity [Fe/H]. The stars of the first data release (Chen et al., 2014) are indicated by filled circles. The remain-ing stars are indicated by open circles.

Figure 2. (Below)The sky distribution of XSL stars in Galactic coordinates. Points are coloured by their 2MASS K-band magnitudes.

Figure 5: The Hertzsprung-Russell diagram of XSL stars, taken from Chen et al.(2014b).

Figure 6: Near-infrared X-shooter spectrum of LTT7987, taken from Moehleret al. (2014).

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The X-shooter Spectral Library is an ambitious project with more than 700 (fi-nal) spectra of stars, which represents almost 10000 raw files. To deal with such anamount of data, a database was created, as described in Chapter 5. Furthermore,a dedicated website was created to release our data to the community.

A significant part of the stars within XSL are cool giants, either from the redgiant branch (RGB) or the asymptotic giant branch (AGB). This is of course nota coincidence. Indeed, these stars have pulsating atmospheres, meaning that thestars actually increase and decrease in size periodically, which leads to changes inthe brightness of the stars. This variability implies that two observations of thesame star one in the optical wavelength range and the other in the near-infraredmight give different results, if the times of observation are different. This issue willaffect the construction of stellar population models. Thankfully with X-shooter,simultaneous observations over a broad wavelength range are now available. Fur-thermore, the (thermally pulsing) AGB is an important phase as stars at thisstage are significant contributors to the near-infrared light of intermediate agestellar populations (1–3 Gyr).

Stars on the AGB have a double-shell burning structure: the carbon-oxygencore is surrounding by a helium (He) burning shell and a further hydrogen (H)burning shell, as shown in Figure 7. As the star evolves on the AGB, the heliumshell runs out of fuel, and the star derives its energy from fusion of hydrogen. Pe-riodically, the helium burning shell ignites violently (usually refers as helium shellflash or thermal pulse) and burns up the helium produced by the hydrogen shellsince the last phase of helium activity. This creates a flash-driven convection zonewhich extends from the He shell almost up the H burning shell. Therefore, theproducts of helium burning (mainly carbon) are mixed throughout the intershellregion. As the flash dies down, the energy deposited in the intershell causes theouter layers to expand and cool, and the H-burning shell is temporarily extin-guished. The base of the convective envelope reaches inward and can penetratebeyond the H/He interface and into the region enriched in carbon. Carbon canthus be mixed to the surface, in the so-called “third dredge-up”.

With each dredge-up episode, the surface ratio of carbon over oxygen (C/O)increases. After a certain number of pulses, it may exceed unity and the star willthen evolve from an oxygen-rich star (C/O < 1) to a carbon-rich star (C/O > 1).This means a change in the surface envelope: the oxides, such as TiO, VO, arereplaced by carbon molecules, such as CN and C2.

Neglecting the diversity of stars available in XSL, I focused during this thesison one type of cool stars: carbon stars. Prior to XSL, only small collections ofC-star spectra existed to represent their diversity.

Thanks to XSL, we doubled the number of carbon star spectra available acrossthe optical and near-infrared spectral range, as shown in Chapter 2. Our sampleincludes stars from the Milky Way and the Magellanic Clouds. Some representativespectra of our sample are displayed in Figure 8.

The main characteristic of our sample is the bimodal behaviour of our starswhen (J −K) is larger than 1.6. At a given near-infrared color, in addition to the“classical” carbon stars, another family of spectra emerge, characterized by thepresence of an absorption feature at 1.53µm and a smoother appearance.

164 English summaryThermally-pulsing AGB stars

Figure 7: Inside view of a thermally pulsing AGB star (credit A. Karakas).

An example of such behaviour is seen when comparing the two orange spectrafrom Fig. 8.

To investigate the culprits of this behaviour, and to determine the fundamentalparameters of our stars (such as the temperature, the metalliticity), we comparedour observed spectra with models. This analysis is summarized in Chapter 3.This comparison also aimed at validating and improving the models. We usedstate-of-the-art hydrostatic carbon-rich models, which means that the pulsationphenomenon and the dust effects are not part of the models. To redden our models,we did not use a standard extinction law, but we chose a polynomial correction,which takes into account the errors related to the flux calibration.

For rather blue carbon stars, we found good fits at intermediate resolution.Overall, the values of the fundamental parameters found in the visible and thenear-infrared agree. However, the uncertainties for the estimated temperaturesremain still high (more than 100 K) because of degeneracy between the parameters.

For the reddest stars, the hydrostatic models are not good anymore, which isexpected. Indeed, those stars are more and more affected by the pulsation effectand by the effects of circumstellar material on the energy distribution (that asimple extinction law cannot reproduce). For those objects, the next step will beto compare them with dynamical models.

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In this thesis, we have developed the X-shooter Spectral Library and somebackstage processes. We have focused our analysis on one spectral type, the carbonstars. Until now, the small number of carbon stars in stellar libraries prevented thereproduction of their diversity in stellar population synthesis. With XSL, we goa step further: this collection extends the previous ones and even shows diversity.A comparison with hydrostatic carbon-rich models was done as a first pass, andthe next step is now to turn to dynamical models, which take into account thepulsation properties. For now, we advise users to average our C-star spectrainstead of using individual ones for stellar population synthesis applications. TheX-shooter Spectral Library is full of stars from various spectral types, and moreanalysis like that in this thesis need to be done, before reaching for the galaxies.

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Figure 8: Representative spectra from our sample of carbon stars, from Gonneauet al. (2015).