Insights on the nature and origin of the LMC bar
As I explained in Section 1.2.1, the LMC bar is an enigmatic structure, with amaz- ing properties: asymmetric and warped geometry, with a centroid off-centre with respect to those of the underlying stellar disc and gas disc, maybe misaligned with respect to and/or located in front of the disc plane. Deciphering the nature and the origin of the so-called LMC bar is one of the main items in the wish list of as- tronomers interested in the LMC. Below, I will discuss three cases: Zaritsky (2004) geometrical solution, the scenario of dynamically-driven bar and the scenario of burst-born bar. In each case, the bar is assumed to be coplanar4.
Zaritsky (2004) proposed an attractive solution — a tri-axial stellar bulge em- bedded in a highly obscuring thin disc — which aims at reproducing all the quoted features. Unfortunately, this solution is not completely satisfactory since it requires a strong reddening (or a very inclined disc which has equivalent effect), which is not supported by several reddening maps of the LMC (see Chapter 3). And if this were the case, one would still have to understand the origin of such a stellar bulge (driven by a dynamical instability in the past or similar to early-type bulges?). In addition to the reddening problem, Cole et al. (2005) measured radial velocities for 373 RGB field stars located in the LMC bar and found a velocity dispersion the velocity dispersion of Cole et al. (2005) LMC bar stars is not uniform with respect to the metallicity: the 5% most metal-rich stars have a velocity disper- sion of ∼ 17 kms−1 while the 5% most metal-poor stars a velocity dispersion of ∼ 41 kms−1, i.e. the most metal-poor population is kinematically hotter than the most metal-rich one and resembles, in that sense, to a bulge or a halo component.
However, as noted by Cole et al. (2005), it is unlikely that the formation of the bar/bulge had preferentially heated metal-poor disc stars.
The LMC bar is often thought as a dynamically-driven bar, i.e. a structure driven by disc instabilities like the one found at the centre of the MW. Physics of bars (formation, dissolution, renewal, effects on the gas etc.) in disc galaxies has been broadly studied: Combes & Sanders (1981), Colin & Athanassoula (1989), Combes et al. (1990), Combes & Elmegreen (1993), Sellwood & Wilkinson (1993), Bournaud & Combes (2002), Regan & Teuben (2004), Kormendy & Kennicutt (2004) etc. For instance, the channelling of gas towards the central regions due to the bar torques is expected but no signatures of the LMC bar have been found in the gas distribution, which strongly weakens the scenario of a dynamically-driven bar. Another possibility could be that, indeed, gas has been driven towards the inner regions and completely turned into stars, which would explain why we do not see a central gas over-density. However it seems unlikely that such a dramatic event would have left such a well-organised disc of gas: indeed, we should anyway expect some signatures in the present-day distributions, like gas-free spaces in or around the central regions. Another interesting point is that bars may be short-lived because of the central mass concentration and destroy themselves within 1-2Gyr (Athanassoula et al., 2005; Bournaud et al., 2005). Yet, Harris & Zaritsky (2009) find that the LMC bar has existed during most of the LMC lifetime (the SFH traces the LMC bar even at old epochs). Simulations (e.g., Bournaud & Combes, 2002) have shown that the formation of bars can be a recurrent process: bars can vanish and be renewed a couple of times over a Hubble time. Therefore, we could imagine that we are observing a second occurrence of a bar structure in the LMC.
However, at each renewal, the bar is expected to be weaker than the previous time; the prominence of the current bar seems to be contradictory (in other words, the previous bar structure should have been even more marked). If we assume that we have a dynamically-driven bar, one still has to explain why it is asymmetric, warped and off-centre. The model 2 in Besla et al. (2012) explains the asymmetric off-centre bar: as the LMC disc is bar unstable, the bar is present from the beginning; it becomes asymmetric off-centre due to a close encounter of the LMC and SMC a few Myr ago.
The LMC inner disc sample: re-analysis of Pompeia et al.
Pompéia et al. (2008) has chemically analysed 59 RGB stars located in the LMC disc (hereafter, referred as the inner disc sample; centre coordinates: (αdisc01, δdisc01) = (5.206 h,−71.222◦)), ∼ 2◦ South of the LMC bar. A low resolution survey (Smecker- Hane et al., Private communication; see also Pompéia et al., 2008) allowed to select LMC inner disc stars in order to sample the low-metallicity tail of the metallicity distribution function. Figures 2.7a, 2.7b and 2.7c show respectively the location of the 114 stars from Smecker-Hane et al. (Private communication) and the 67 stars from Pompéia et al. (2008) on a (V−I), I CMD, the metallicity distribution function and the radial velocity distribution of these two samples. As for the LMC bar stars, the LMC inner disc stars have been observed with GIRAFFE using the setups HR11, HR13 and HR145. In order to homogeneously compare the LMC bar and inner disc samples, I will re-analyse Pompéia et al. (2008) sample (re-determination of both stellar parameters and abundances). To this end, in Chapters 3 and 4, I will use equivalent width (EW) and (already) reduced spectra of Pompéia et al. (2008) and apply my pipelines. Table 2.6 gives, for the LMC disc stars, their identifiers, VI magnitudes from Smecker-Hane et al. (2002), JHK magnitudes from Skrutskie et al. (2006), [Fe/H]CaT and vrad,CaT from Smecker-Hane et al. (Private communication) (and corresponding 1-σ errors).
Sky continuum and emission lines
The sky background Ssky is an additional (polluting) light, recorded simultaneously to the science light; it has a continuum and emission lines component. The Earth atmosphere is the main contributor to the sky background. Indeed, even during the night, the Earth atmosphere radiates and this atmosphere brightness is due to: thermal continuum, airglow (upper atmosphere), emission lines (de-excitation of atoms and molecules in the Earth atmosphere) etc. Other sources responsible for the sky background are: moonlight, zodiacal light (sunlight scattered or reflected by interplanetary dust particles), integrated light of faint objects (non-resolved stars, distant galaxies), and diffuse Galactic light (Galactic starlight scattered or reflected by interstellar dust).
The sky background is an additive effect and has to be accurately removed from the recorded spectra to allow accurate abundance measurements. The sky subtraction is a tricky question. To isolate the sky component, we have to observe a blank region of the sky, i.e. a region free of stars. Because of possible spatial dependency of the sky emission, this region should be close to the science target to sample the same portion of the atmosphere. And due to possible time-dependency, we should record the sky spectrum simultaneously to the science spectrum. For our observations, the first constraint (spatial dependency) is released since the sky emission can be considered constant over the GIRAFFE field of view (θ ≈ 25′).
Concerning the time-dependency, in multi-object spectroscopy, it is always possible to devote a number of fibres to record the sky spectrum simultaneously to the science exposure.
As described in Section 2.1.2, in our observing program we allocated around 10 to 20 fibres to sky positions. As for the bias, dark or flat-field frames, the question of noise arises: the sky fluxes are marred by various sources of noise, especially the photon noise, and we have to minimise this, otherwise we will decrease the S/N ratio of the stellar spectra. One possibility is to carefully combine the different sky spectra and form a master sky spectrum. I adopted the following strategy. I visually sorted out the sky spectra to check for their quality and discarded those showing spectral contamination or defects: jump in fluxes due to polluting light (stellar light, CCD glow, simultaneous calibration lamp) or a CCD defect. After this quality selection, we ended up with a handful of sky spectra (at least five to eight) in most cases. As the selected sky spectra exhibit the same continuum level (testifying of the spatial stability of the atmosphere layers), I averaged them with k-σ clipping rejection and subtracted the resulting master sky to each stellar spectrum. This procedure was applied to each stellar spectrum (i.e. to each observation of the 113 bar stars and for each setup). Thus, after this step, we are left with: ˜ S⋆(λ) = tatm(λ)S⋆(λ).
Table of contents :
1.1 The Large Magellanic Cloud in a cosmological context
1.1.1 The Large Magellanic Cloud within the Local Group
1.1.2 LMC-like galaxies in a ΛCDM context
1.2 The Large Magellanic Cloud: morphology, interaction history and chemical evolution
1.2.1 Morphology and kinematics
1.2.2 Stellar populations: observational facts
220.127.116.11 Globular clusters
18.104.22.168 Field stars
22.214.171.124 Detailed chemical abundances
1.2.3 Possible chemical evolution scenarios
1.2.4 Possible dynamical scenarios
1.2.5 Insights on the nature and origin of the LMC bar
1.3 Aims and structure of this thesis work
2 Observations & data processing
2.1.1 The FLAMES/GIRAFFE spectrograph
2.1.2 The LMC bar sample
2.1.3 The LMC inner disc sample: re-analysis of Pompeia et al.
2.2 Data processing
2.2.1 Extraction of astronomical spectra
126.96.36.199 Instrumental signatures
188.8.131.52 Spectra extraction
184.108.40.206 Wavelength calibration
220.127.116.11 Putting it all together
2.2.2 Effects of the atmosphere
18.104.22.168 Sky continuum and emission lines
22.214.171.124 Telluric absorption bands
2.2.3 Noise and signal-to-noise ratio measurements
2.2.4 Radial velocities measurements
126.96.36.199 Method to determine the radial velocities
188.8.131.52 Radial velocity measurements for the LMC bar sample
184.108.40.206 Re-determination of the radial velocities for the LMC inner disc sample
2.2.5 Co-addition of spectra and signal-to-noise ratio
2.3 Arcturus as a benchmark star
2.3.1 Principle and aims of differential chemical analysis
2.3.2 Preparation of Arcturus GIRAFFE-like spectra
2.3.3 Discussion on signal-to-noise ratio measurements
2.4 Large tables
3 Stellar parameters determination
3.2 Effective temperature
3.2.2 How to determine effective temperature?
3.2.3 Photometric temperature of our LMC stars
3.3 Surface gravity
3.3.2 How to determine surface gravity?
3.3.3 Isochrone gravities of our LMC stars
3.4 Overall metallicity
3.4.2 Metallicity of our LMC bar stars
3.5 Microturbulence velocity
3.5.2 How to determine microturbulence velocity?
3.5.3 Microturbulence velocity and metallicity of our LMC stars
3.6 Choice of the reddening
3.7 [Fe/H]CaT vs. [Fe/H]spec
3.8 Large tables
4 Abundance analysis
4.1 From absorption lines to chemical abundances
4.1.2 Radiative transfer
4.1.3 Line profile
220.127.116.11 Microscopic effects
18.104.22.168 Macroscopic effects
4.1.4 Curve of growth
4.2 Abundance analysis of our LMC bar and disc stars
4.2.1 Procedures of abundance measurements
4.2.2 Line lists: compilation and calibration
4.2.3 Application to Arcturus
4.2.4 The LMC bar sample
4.2.5 The LMC disc sample
22.214.171.124 Signal-to-noise ratio for our LMC inner disc stars .
4.3 Error budget
4.3.1 Abundance measurement
4.3.2 Atomic data and line modelling
4.3.3 Stellar parameters
4.4 Correlations between abundance ratios and stellar parameters
5 The chemical history of the Large Magellanic Cloud
5.1 An introduction to galaxy chemical evolution
5.1.1 Stars as chemical element factories
5.1.2 Nucleosynthesis of elements lighter than iron
126.96.36.199 Main sequence evolution
188.8.131.52 Post-main sequence evolution
5.1.3 Iron-peak elements production by type Ia supernova
5.1.4 Nucleosynthesis of elements heavier than the iron-peak
184.108.40.206 AGB nucleosynthesis: s-process
220.127.116.11 Explosive nucleosynthesis: r-process
5.1.5 From stellar nucleosynthesis to galactic chemical evolution .
5.2 The chemical evolution of the LMC
5.2.1 A slow chemical evolution
5.2.2 Is the LMC IMF different?
5.2.3 Do we need prompt type Ia supernovae?
5.3 Chemical anomalies: new lights on nucleosynthesis models
5.4 A new picture for the formation of the LMC bar
6 Conclusion & future works
6.1 Main developments for this thesis work
6.2 Main results of this thesis work
6.3 Future works
A Final line lists
B Final abundances for LMC bar stars
C Final abundances for LMC bar stars