Intermediate- and high-velocity clouds (IVCs, HVCs) are a potential source of fuel for star formation in the Milky Way (MW), but their origins and fates depend sensitively on their distances. We search for IVCs and HVCs in HST high-resolution ultraviolet spectra of 55 halo stars at vertical heights $|z|\gtrsim \,1$ kpc. We show that IVCs (40 ≤ |$v$LSR| < 90 ${\rm km\, s}^{-1}$) have a high detection rate – the covering factor, fc – that is about constant (fc = 0.90 ± 0.04) from $z$ = 1.5 to 14 kpc, implying IVCs are essentially confined to |$z$| ≲ 1.5 kpc. For the HVCs (90 ≤ |$v$LSR| ≲ 170 ${\rm km\, s}^{-1}$), we find fc increases from fc ≃ 0.14 ± 0.10 at |$z$| ≲ 2–3 kpc to fc = 0.60 ± 0.15 at 6 ≲ |$z$| ≲ 14 kpc, the latter being similar to that found towards QSOs. In contrast, the covering factor of very high-velocity clouds (VHVCs; |$v$LSR| ≳ 170 ${\rm km\, s}^{-1}$) is $f_c \lt 0.04$ in the stellar sample compared to 20 per cent towards QSOs, implying these clouds must be at d ≳ 10–15 kpc (|$z$| ≳ 10 kpc). Gas clouds with |$v$LSR| > 40 ${\rm km\, s}^{-1}$ at |b| ≳ 15° have therefore |$v$LSR| decreasing with decreasing |$z$|. Our findings are consistent with a Galactic rain and/or fountain origin for these clouds. In the latter scenario, VHVCs may mostly serve as fuel for the MW halo. In view of their high covering factors and since all the IVCs and some HVCs are found in the thick disc, they appear good candidates as gas reservoirs to help sustain star formation in the MW.
Carbon-enhanced metal-poor (CEMP) stars comprise almost a third of stars with [Fe/H] < −2, although their origins are still poorly understood. It is highly likely that one sub-class (CEMP-s stars) is tied to mass-transfer events in binary stars, while another sub-class (CEMP-no stars) are enriched by the nucleosynthetic yields of the first generations of stars. Previous studies of CEMP stars have primarily concentrated on the Galactic halo, but more recently they have also been detected in the thick disc and bulge components of the Milky Way. Gaia DR3 has provided an unprecedented sample of over 200 million low-resolution (R ≈ 50) spectra from the BP and RP photometers. Training on the CEMP catalogue from the SDSS/SEGUE database, we use XGBoost to identify the largest all-sky sample of CEMP candidate stars to date. In total, we find 58 872 CEMP star candidates, with an estimated contamination rate of 12 per cent. When comparing to literature high-resolution catalogues, we positively identify 60–68 per cent of the CEMP stars in the data, validating our results and indicating a high completeness rate. Our final catalogue of CEMP candidates spans from the inner to outer Milky Way, with distances as close as r ∼ 0.8 kpc from the Galactic centre, and as far as r > 30 kpc. Future higher resolution spectroscopic follow-up of these candidates will provide validations of their classification and enable investigations of the frequency of CEMP-s and CEMP-no stars throughout the Galaxy, to further constrain the nature of their progenitors.
more » « less- NSF-PAR ID:
- 10422808
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 523
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 4049-4066
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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