Stellar loci IV. red giant stars

In the fourth paper of this series, we present the metallicity-dependent Sloan Digital Sky Survey (SDSS) stellar color loci of red giant stars, using a spectroscopic sample of red giants in the SDSS Stripe 82 region. The stars span a range of 0.55 – 1.2 mag in color g – i, –0.3 – –2.5 in metallicity [Fe/H], and have values of surface gravity log g smaller than 3.5 dex. As in the case of main-sequence (MS) stars, the intrinsic widths of loci of red giants are also found to be quite narrow, a few mmag at maximum. There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars. The colors of red giants are less sensitive to metallicity than those of MS stars. With good photometry, photometric metallicities of red giants can be reliably determined by fitting the u – g, g – r, r – i, and i – z colors simultaneously to an accuracy of 0.2 – 0.25 dex, comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10. By comparing fitting results to the stellar loci of red giants and MS stars, we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry. The technique achieves completeness of ∼70 per cent and efficiency of ∼80 per cent in selecting metal-poor red giant stars of [Fe/H] ≤ –1.2. It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.

Chapter 5 Dark Matter and New Physics Beyond the Standard Model with LHAASO

In order to reveal the nature of dark matter, it is crucial to detect its non-gravitational interactions with the standard model particles. The traditional dark matter searches focused on the so-called weakly interacting massive particles. However, this paradigm is strongly constrained by the null results of current experiments with high precision. Therefore there is a renewed interest of searches for heavy dark matter particles above TeV scale. The Large High Altitude Air Shower Observatory (LHAASO) with large effective area and strong background rejection power is very suitable to investigate the gamma-ray signals induced by dark matter annihilation or decay above TeV scale. In this document, we review the theoretical motivations and background of heavy dark matter. We review the prospects of searching for the gamma-ray signals resulted from dark matter in the dwarf spheroidal satellites and Galactic halo for LHAASO, and present the projected sensitivities. We also review the prospects of searching for the axion-like particles, which are a kind of well motivated light pseudo-scalars, through the LHAASO measurement of the very high energy gamma-ray spectra of astrophysical sources.

NICER Study of Pulsed Thermal X-Rays from Calvera: A Neutron Star Born in the Galactic Halo?

Calvera (1RXS J141256.0+792204) is an isolated neutron star detected only through its thermal X-ray emission. Its location at high Galactic latitude (b = +37°) is unusual if Calvera is a relatively young pulsar, as suggested by its spin period (59 ms) and period derivative (3.2 × 10−15 s s−1). Using the Neutron Star Interior Composition Explorer, we obtained a phase-connected timing solution spanning four years, which allowed us to measure the second derivative of the frequency ν ̈ = − 2.5 × 10 − 23 Hz s−2 and to reveal timing noise consistent with that of normal radio pulsars. A magnetized hydrogen atmosphere model, covering the entire star surface, provides a good description of the phase-resolved spectra and energy-dependent pulsed fraction. However, we found that a temperature map more anisotropic than that produced by a dipole field is required, with a hotter zone concentrated toward the poles. By adding two small polar caps, we found that the surface effective temperature and that of the caps are ∼0.1 and ∼0.36 keV, respectively. The inferred distance is ∼3.3 kpc. We confirmed the presence of an absorption line at 0.7 keV associated with the emission from the whole star surface, difficult to interpret as a cyclotron feature and more likely originating from atomic transitions. We searched for pulsed γ-ray emission by folding seven years of Fermi-LAT data using the X-ray ephemeris, but no evidence for pulsations was found. Our results favor the hypothesis that Calvera is a normal rotation-powered pulsar, with the only peculiarity of being born at a large height above the Galactic disk.

The H i Column Density Distribution of the Galactic Disk and Halo

We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm−2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude ∣b∣ ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ–Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = − 1.01 − 0.14 + 0.15 and β HVC = − 1.05 − 0.06 + 0.07 . Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with −0.88 ± 0.40 M ⊙ yr−1 of HVC inflow versus ≈0.20 ± 0.10 M ⊙ yr−1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.

RR Lyrae Stars in the Newly Discovered Ultra-faint Dwarf Galaxy Centaurus I*

We report the detection of three RR Lyrae (RRL) stars (two RRc and one RRab) in the ultra-faint dwarf (UFD) galaxy Centaurus I (Cen I) and two Milky Way (MW) δ Scuti/SX Phoenicis stars based on multi-epoch giz DECam observations. The two RRc stars are located within two times the half-light radius (r h ) of Cen I, while the RRab star (CenI-V3) is at ∼6 r h . The presence of three distant RRL stars clustered this tightly in space represents a 4.7σ excess relative to the smooth distribution of RRL in the Galactic halo. Using the newly detected RRL stars, we obtain a distance modulus to Cen I of μ 0 = 20.354 ± 0.002 mag (σ = 0.03 mag), a heliocentric distance of D ⊙ = 117.7 ± 0.1 kpc (σ = 1.6 kpc), with systematic errors of 0.07 mag and 4 kpc. The location of the Cen I RRL stars in the Bailey diagram is in agreement with other UFD galaxies (mainly Oosterhoff II). Finally, we study the relative rate of RRc+RRd (RRcd) stars (f cd) in UFD and classical dwarf galaxies. The full sample of MW dwarf galaxies gives a mean of f cd = 0.28. While several UFD galaxies, such as Cen I, present higher RRcd ratios, if we combine the RRL populations of all UFD galaxies, the RRcd ratio is similar to the one obtained for the classical dwarfs (f cd ∼ 0.3). Therefore, there is no evidence for a different fraction of RRcd stars in UFD and classical dwarf galaxies.

Machine-learned dark matter subhalo candidates in the 4FGL-DR2: search for the perturber of the GD-1 stream

The detection of dark matter subhalos without a stellar component in the Galactic halo remains a challenge. We use supervised machine learning to identify high-latitude gamma-ray sources with dark matter-like spectra among unassociated gamma-ray sources in the 4FGL-DR2. Out of 843 4FGL-DR2 unassociated sources at |b| ≥ 10°, we select 73 dark matter subhalo candidates. Of the 69 covered by the Neil Gehrels Swift Observatory (Swift), 17 show at least one X-ray source within the 95% LAT error ellipse and 52 where we identify no new sources. This latest inventory of dark subhalos candidates allows us to investigate the possible dark matter substructure responsible for the perturbation in the GD-1 stellar stream. In particular, we examine the possibility that the alleged GD-1 dark subhalo may appear as a 4FGL-DR2 gamma-ray source from dark matter annihilation into Standard Model particles.

Nitrogen evolution in the halo, thick disc, thin disc, and bulge of the Galaxy

ABSTRACT We study the evolution of nitrogen (N) in the Galactic halo, thick disc, thin disc, and bulge by comparing detailed chemical evolution models with recent observations. The models used in this work have already been constrained to explain the abundance patterns of α-elements and the metallicity distribution functions of halo, disc, and bulge stars; here, we adopt them to investigate the origin and evolution of N in the different Galactic components. First, we consider different sets of yields and study the importance of the various channels proposed for N production. Secondly, we apply the reference models to study the evolution of both the Galactic discs and bulge. We conclude that: i) primary N produced by rotating massive stars is required to reproduce the plateau in log(N/O) and [N/Fe] ratios at low metallicity, as well as the secondary and primary production from low- and intermediate-mass stars to reproduce the data of the thin disc; ii) the parallel model can provide a good explanation of the evolution of N abundance in the thick and thin discs, and we confirm that the thick disc has evolved much faster than the thin disc, in agreement with the results from the abundance patterns of other chemical elements; and iii) finally, we present new model predictions for N evolution in the Galactic bulge, and we show that the observations in bulge stars can be explained if massive stars rotate fast during the earliest phases of Galactic evolution, in agreement with findings from the abundance pattern of carbon.