Carbon measurements in two ultra-faint dwarf galaxies: Grus II and Tucana IV

TL;DR

The paper investigates carbon-enhanced metal-poor stars in two ultra-faint dwarf galaxies, Grus II and Tucana IV, to probe the earliest chemical enrichment by Pop III stars. It combines FLAMES/GIRAFFE spectroscopy targeting RGB and HB stars with Gaia DR3 astrometry to identify members and measure [Fe/H] and [C/Fe] via the Ca II triplet and CH band. The analysis uncovers several CEMP-no stars, including three at [Fe/H]~−3 with [C/Fe] > +1 in Grus II and one in Tucana IV, indicating that faint Pop III supernovae left a lasting imprint in these systems. The results suggest elevated CEMP fractions at low metallicity in UFDs, offering constraints on early star-formation and chemical enrichment in the smallest galactic building blocks.

Abstract

The ultra-faint dwarf galaxies (UFDs) are some of the oldest and most metal-poor environments in the Local Group. In particular, they are predicted to host the first stars (only H and He) that lit up in our Universe. No metal-free stars have been found to date, but their chemical products can be observed on the surfaces of the ancient second-generation stars such as the carbon-enhanced metal-poor stars (CEMP-no, [C/Fe]>+0.7).} However, in each UFD there are only a few stars bright enough for spectroscopic follow-up, therefore it is crucial to study as many of these systems as possible. Here we follow up stars belonging to two recently discovered UFDs, Grus II and Tucana IV. The spectra analyzed were obtained with the multi-object spectrograph FLAMES/Giraffe at the Very Large Telescope (VLT). This includes spectra in two wavelength ranges: red spectra around the CaII triplet (8498 Å, 8542 Å, 8662 Å) used to derive radial velocity and [Fe/H], and blue spectra covering the CH band at ~ 4300 Å. In total, we analyzed 21 spectra of member candidates for Grus II and 17 for Tucana IV, including both Red Giant Branch (RGB) and Horizontal Branch (HB) stars. We identified 13 members in Grus II (thereof 8 RGB stars) and 7 members in Tucana IV (thereof 3 RGB stars). Among the RGB stars in Grus II, we found three CEMP-no stars at [Fe/H]~-3 and [C/Fe]>+1 and two CEMP-no stars at slightly higher [Fe/H] and [C/Fe]>+0.7. In Tucana IV, we found one CEMP-no star ([Fe/H]=-2.75 and [C/Fe] = +0.83). This project, along with future investigations of CEMP stars in UFDs, allows us to study the impact of the first stars in these ancient and primitive systems and consequently the first chemical enrichment that occurred in the Universe.

Figures (18)

• Figure 1: Example of spectra of our target stars. Top: star 6 in Grus II, a hot HB star whose spectrum is dominated by Paschen lines. Middle: the CaII triplet (2nd panel) and the CH band (3rd panel) spectra of the RGB star 26 in Grus II, Bottom: CH band spectrum of the RGB star 4 in Grus II.
• Figure 2: Color-magnitude diagram of Grus II (left panel) and Tucana IV (right panel). The star symbols indicate our FLAMES/GIRAFFE targets, larger symbols are the member stars determined in Section \ref{['vrad text']}, and black small circles represent possible members from the catalog by battaglia_gaia_2022 based on Gaia eDR3 measurements. Numbers are the star IDs (\ref{['tab par G']} and \ref{['tab par T']}). \ref{['isochrones']} also shows the CMD with isochrones of different metallicities overplotted.
• Figure 3: The CaII triplet of the star 26 in Grus II. The black line is the observed spectrum, the magenta line is the synthetic spectrum best fit to the radial velocity of the star.
• Figure 4: Radial velocity distribution of Grus II (left panel) and Tucana IV (right panel). The black bars represent the non-members, the colored ones are the members. The shaded bars are the HB members, which do not present any significant difference or offset from the RGB stars in radial velocity. We only include stars with red spectra available, but we note that the HB stars 1 and 14 in Grus II are members based only on measurements from blue spectra (\ref{['tab SN gru']}).
• Figure 5: Proper motions of the stars in Grus II and Tucana IV from Gaia DR3. The stars are color-coded according to the membership probability based on battaglia_gaia_2022. The red cross is the systemic proper motion battaglia_gaia_2022, the red ellipse corresponds to 1$\sigma$, which is the error on the mean proper motions.