Below is an overview of observation programmes of which I am the PI. Both raw and reduced data from VLT observations can be obtained through the ESO archive.

The early Universe next door: a MUSE view of the extremely low-metallicity galaxy Sextans A

Telescope/Instrument: VLT/MUSE
Program ID: 108.221U
Time allocated: 12 hours (Service Mode)
Status: In progress

To constrain the properties of star-forming galaxies in the early Universe it is critical to understand the evolution and feedback of low-metallicity massive stars, yet very few observational constraint exist below the metallicity of the SMC. Here, we target two fields in one of the lowest metallicity dwarf galaxies in the Local Group, Sextans A (Z ≈ 1/15 Zsun). We will construct the first extensive spectral catalog of massive stars at sub-SMC metallicity, and constrain the stellar properties. The results, unprecedented at the low-metallicity regime, will include a lower limit of the binary fraction, the distribution of rotational velocities of the O-type stars, and a search for specific channels and (late) stages of metal-poor evolution. This allows us to test, for the first time, tailored models of the evolution of massive stars at very low metallicity which, in turn, provide updated estimates of feedback for early-Universe galaxies that are one of the main science drivers of JWST.

Composite image of Sextans A (left; blue = GALEX NUV, green = V, red = Hα) indicating the fields to be observed (thick squares). The colour-magnitude diagram (middle) shows all stars in Sextans A (black dots), and the stars in the respective fields (blue: ‘bubble’, green: ‘field’). LMC and SMC WR stars (red stars) and the LBV (V39)and WO star (DR1) from IC1613 would occupy inconspicuous locations in this diagram. The right panel shows how the galaxy’s internal reddening can lead to severely underestimated stellar masses from photometry alone. Red and black squares mark 4 O-type stars confirmed by spectroscopy before and after reddening correction, illustrating that the most massive stars may be inconspicuous in the CMD.

The role of episodic mass loss in the evolution of massive stars

Telescope/Instrument: VLT/FORS2
Program ID: 0105.D-0486
Time allocated: 39.3 hours (Service Mode)
Status: In progress

A dominant factor in the evolution of massive stars is the amount of mass lost during their lifetime. Classical line-driven winds are insufficient to explain the observed masses of evolved massive stars, and hence additional mass loss is needed. One of the alternative mechanisms is episodic mass loss, such as η Car-like eruptions. This proposal aims to investigate the incidence and relative importance of episodic mass loss by observing a large number (~1000) of dusty, evolved massive star candidates in nearby galaxies with VLT/FORS, spanning a large range of metallicities. The obtained spectra will be classified and scrutinised for signatures of past or current mass loss events. We will identify dusty, evolved massive stars that have undergone episodic mass loss and assess their occurrence as a function of metallicity. The observations will increase the number of spectroscopically identified massive stars in nearby Southern galaxies other than the Magellanic Clouds by a factor ~5.

Pre-imaging of one of the fields in NGC55 (colorscale added).

Elucidating the nature of nitrogen-rich slowly rotating massive stars in 30 Doradus

Telescope/Instrument: VLT/UVES
Program ID: 0100.D-0192
Time allocated: 1.6 nights (designated Visitor Mode)
Status: Completed, publication in preparation

Nitrogen enrichment in the atmospheres of massive stars through rotational mixing is an important prediction from evolutionary models. Two previous large programs (VLT-FLAMES Survey of Massive Stars and VLT-FLAMES Tarantula Survey) have identified a group of slowly rotating, nitrogen-enriched massive stars that cannot be explained with our current understanding of rotational mixing. However, the existing data, with limited wavelength coverage, and moderate spectral resolution and signal-to-noise, have not allowed to elucidate their nature. Here we propose to obtain high-resolution, high-S/N UVES data covering the entire optical wavelength range for a representative subsample of these stars. By performing a detailed quantitative analysis we aim to find clues to the origin of these stars.

Overview of spectra from the blue UVES arm.

Investigating the nature of WO stars: hot WC stars or a separate evolutionary stage?

Telescope/Instrument: VLT/X-Shooter
Program ID: 093.D-0591
Time allocated: 2 hours (Service Mode)
Status: Completed, published

With only eight known members, WO stars are the rarest type of Wolf-Rayet stars. They are in an advanced stage of helium burning and may represent the last stage in massive star evolution. Despite their importance, WO stars are still poorly understood. There are two competing theories on how WO stars compare to the more common WC stars. Firstly, they could represent a more advanved evolutionairy stage and have an enhanced oxygen and carbon abundance. Alternatively, they could be the hot end of the WC sequence, and not necesarily further into their helium-burning stage. To investigate their nature, we aim to perform quantitative X-Shooter spectroscopy of all known WO stars. Six of them have already been observed in periods P88 and P91. We now propose to observe the remaining two stars, which have critical properties for our sample: WR30a, the only Galactic WO in a binary system, and WR142, the only single WO with known, unusually hard, X-ray emission.

Overview of WO spectra in the UVB-VIS wavelength range.

Quantitative spectroscopy of massive stars in the low-metallicity galaxy NGC 3109: testing the mass-loss versus metallicity relation

Telescope/Instrument: VLT/X-Shooter
Program ID: 090.D-0212
Time allocated: 2 nights (GTO; Visitor Mode)
Status: Completed, published

The mass loss of massive stars is driven by radiation pressure on metallic ion lines, and is therefore predicted to scale with metallicity. This prediction has been verified for stellar winds in the Galaxy and Magellanic Clouds. To test this prediction at lower metallicities, we have obtained and quantitatively analyzed X-Shooter spectra of a sample of stars in three low-metallicity dwarf galaxies (at Z = 0.14 Zsun). Surprisingly, the wind strengths tend to be greater than predicted, and in two cases are in direct conflict with theory. To further investigate this potential breakdown of radiation-driven wind theory, we propose to observe six more O-type stars in the low-metallicity galaxy NGC 3109, hosting the star with the largest discrepancy with model predictions. If this breakdown of the theory is confirmed, this has strong implications for our understanding of low-metallicity stars and the early Universe, and poses problems to the single-star long-duration gamma-ray burst channel.

Overview of low-metallicity O-type stars and their best-fit Fastwind models.

Quantitative spectroscopy of massive stars in low metallicity environments: IC 1613

Telescope/Instrument: VLT/X-Shooter
Program ID: 088.D-0181
Time allocated: 1 night (GTO; Visitor Mode)
Status: Completed, published

Calibrating massive star evolutionary models up to the pre-supernova stage requires to pinpoint the evolutionary status, and thus the physical and wind parameters, of the most massive stars at various evolutionary stages. Using the unprecedented opportunity offered by X-shooter, we have initiated the first quantitative spectroscopic analysis of some of the most massive stars known in the Local Group. In the first phase of this project we have, among other observations, secured four spectra of stars in the low-metallicity (Z ~ 1/10 Zsun) dwarf galaxy IC 1613. The theory of radiation driven winds predicts the mass loss rates of these stars to be much lower than those of their Galactic counterparts. Surprisingly, we find that the winds are of comparable strength. To investigate the nature of this discrepancy in more detail, we now aim to increase our sample of stars in IC 1613 (this proposal) and in NGC 3109 (accompanying open time proposal), which has a comparable metallicity.

Wind-momentum versus luminosity diagram for the low-metallicity O-type stars.