| 1. |
- Alfaro-Cuello, M., et al.
(författare)
-
A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. II. Kinematic Characterization of the Stellar Populations
- 2020
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 892:1
-
Tidskriftsartikel (refereegranskat)abstract
- The Sagittarius dwarf spheroidal galaxy is in an advanced stage of disruption but still hosts its nuclear star cluster (NSC), M54, at its center. In this paper, we present a detailed kinematic characterization of the three stellar populations present in M54: Young metal-rich (YMR); intermediate-age metal-rich (IMR); and old metal-poor (OMP), based on the spectra of 6500 individual M54 member stars extracted from a large Multi-Unit Spectroscopic Explorer (MUSE)/Very Large Telescope data set. We find that the OMP population is slightly flattened with a low amount of rotation (0.8 km s-1) and with a velocity dispersion that follows a Plummer profile. The YMR population displays a high amount of rotation (5 km s-1) and a high degree of flattening, with a lower and flat velocity dispersion profile. The IMR population shows a high but flat velocity dispersion profile, with some degree of rotation (2 km s-1). We complement our MUSE data with information from Gaia DR2 and confirm that the stars from the OMP and YMR populations are comoving in 3D space, suggesting that they are dynamically bound. While dynamical evolutionary effects (e.g., energy equipartition) are able to explain the differences in velocity dispersion between the stellar populations, the strong differences in rotation indicate different formation paths for the populations, as supported by an N-body simulation tailored to emulate the YMR-OMP system. This study provides additional evidence for the M54 formation scenario proposed in our previous work, where this NSC formed via GC accretion (OMP) and in situ formation from gas accretion in a rotationally supported disk (YMR).
|
|
| 2. |
- Arabsalmani, M., et al.
(författare)
-
Local Starburst Conditions and Formation of GRB 980425/SN 1998bw within a Collisional Ring
- 2020
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 899:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present the first spatially resolved study of molecular gas in the vicinity of a gamma-ray burst (GRB), using CO(2-1) emission-line observations with the Atacama Large Millimetre Array at ∼50 pc scales. The host galaxy of GRB 980425 contains a ring of high column density H i gas, which is likely to have formed due to a collision between the GRB host and its companion galaxy, within which the GRB is located. We detect 11 molecular gas clumps in the galaxy, 7 of which are within the gas ring. The clump closest to the GRB position is at a projected separation of ∼280 pc. Although it is plausible that the GRB progenitor was ejected from clusters formed in this clump, we argue that the in situ formation of the GRB progenitor is the most likely scenario. We measure the molecular gas masses of the clumps and find them to be sufficient for forming massive star clusters. The molecular gas depletion times of the clumps show a variation of ∼2 dex, comparable with the large variation in depletion times found in starburst galaxies in the nearby universe. This demonstrates the presence of starburst modes of star formation on local scales in the galaxy, even while the galaxy as a whole cannot be categorized as a starburst based on its global properties. Our findings suggest that the progenitor of GRB 9802425 was originated in a young massive star cluster formed in the starburst mode of star formation.
|
|
| 3. |
- Calamida, Annalisa, et al.
(författare)
-
The Not so Simple Stellar System ω Cen. II. Evidence in Support of a Merging Scenario
- 2020
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 891:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present multiband photometry covering ∼5° × 5° across ω Cen collected with the Dark Energy Camera on the 4 m Blanco telescope, combined with Hubble Space Telescope and Wide Field Imager data for the central regions. The unprecedented photometric accuracy and field coverage allows us to confirm the different spatial distribution of blue and red main-sequence stars, and of red giant branch (RGB) stars with different metallicities. The ratio of the number of blue to red main-sequence stars shows that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one, with the frequency of blue main-sequence stars increasing at a distance of ∼20′ from ω Cen's center. Similarly, the more metal-rich RGB stars show a more extended spatial distribution compared to the more metal-poor ones in the outskirts of the cluster. Moreover, the centers of the distributions of metal-rich and metal-poor RGB stars are shifted in different directions with respect to the geometrical center of ω Cen. We constructed stellar density profiles for the blue and red main-sequence stars; they show that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one in the outskirts of ω Cen, confirming the results based on the number ratio. We also computed the ellipticity profile of ω Cen, which has a maximum value of 0.16 at a distance of ∼8′ from the center, and a minimum of 0.05 at ∼30′; the average ellipticity is ∼0.10. The circumstantial evidence presented in this work suggests a merging scenario for the formation of the peculiar stellar system ω Cen.
|
|
| 4. |
- Dainotti, Maria Giovanna, et al.
(författare)
-
Predicting the Redshift of γ-Ray-loud AGNs Using Supervised Machine Learning
- 2021
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 920:2
-
Tidskriftsartikel (refereegranskat)abstract
- Active galactic nuclei (AGNs) are very powerful galaxies characterized by extremely bright emissions coming from their central massive black holes. Knowing the redshifts of AGNs provides us with an opportunity to determine their distance to investigate important astrophysical problems, such as the evolution of the early stars and their formation, along with the structure of early galaxies. The redshift determination is challenging because it requires detailed follow-up of multiwavelength observations, often involving various astronomical facilities. Here we employ machine-learning algorithms to estimate redshifts from the observed γ-ray properties and photometric data of γ-ray-loud AGNs from the Fourth Fermi-LAT Catalog. The prediction is obtained with the Superlearner algorithm using a LASSO-selected set of predictors. We obtain a tight correlation, with a Pearson correlation coefficient of 71.3% between the inferred and observed redshifts and an average Δz norm = 11.6 10-4. We stress that, notwithstanding the small sample of γ-ray-loud AGNs, we obtain a reliable predictive model using Superlearner, which is an ensemble of several machine-learning models.
|
|
| 5. |
- Griffith, Emily, et al.
(författare)
-
The Similarity of Abundance Ratio Trends and Nucleosynthetic Patterns in the Milky Way Disk and Bulge
- 2021
-
Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 909:1
-
Tidskriftsartikel (refereegranskat)abstract
- We compare abundance ratio trends in a sample of similar to 11,000 Milky Way bulge stars (R-GC < 3 kpc) from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to those of APOGEE stars in the Galactic disk (5 kpc < R-GC < 11 kpc). We divide each sample into low-Ia (high-[Mg/Fe]) and high-Ia (low-[Mg/Fe]) populations, and in each population, we examine the median trends of [X/Mg] versus [Mg/H] for elements X = Fe, O, Na, Al, Si, P, S, K, Ca, V, Cr, Mn, Co, Ni, Cu, and Ce. To remove small systematic trends of APOGEE abundances with stellar log(g), we resample the disk stars to match the log(g) distributions of the bulge data. After doing so, we find nearly identical median trends for low-Ia disk and bulge stars for all elements. High-Ia trends are similar for most elements, with noticeable (0.05-0.1 dex) differences for Mn, Na, and Co. The close agreement of abundance trends (with typical differences less than or similar to 0.03 dex) implies that similar nucleosynthetic processes enriched bulge and disk stars despite the different star formation histories and physical conditions of these regions. For example, we infer that differences in the high-mass slope of the stellar initial mass function between disk and bulge must have been less than or similar to 0.30. This agreement, and the generally small scatter about the median sequences, means that one can predict all of a bulge star's APOGEE abundances with good accuracy knowing only its measured [Mg/Fe] and [Mg/H] and the observed trends of disk stars.
|
|
| 6. |
- Hasselquist, Sten, et al.
(författare)
-
APOGEE Chemical Abundance Patterns of the Massive Milky Way Satellites
- 2021
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 923:2
-
Tidskriftsartikel (refereegranskat)abstract
- The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf Galaxy (Sgr), Fornax (Fnx), and the now fully disrupted Gaia Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [α/Fe]-[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the Magellanic Clouds (MCs) observed by Nidever et al. in the α-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3-4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier (∼5-7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.
|
|
| 7. |
- Johansen, Anders, et al.
(författare)
-
Transport, destruction, and growth of pebbles in the gas envelope of a protoplanet
- 2020
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 903:2
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze the size evolution of pebbles accreted into the gaseous envelope of a protoplanet growing in a protoplanetary disk, taking into account collisions driven by the relative sedimentation speed as well as the convective gas motion. Using a simple estimate of the convective gas speed based on the pebble accretion luminosity, we find that the speed of the convective gas is higher than the sedimentation speed for all particles smaller than 1 mm. This implies that both pebbles and pebble fragments are strongly affected by the convective gas motion and will be transported by large-scale convection cells both toward and away from the protoplanet's surface. We present a simple scheme for evolving the characteristic size of the pebbles, taking into account the effects of erosion, mass transfer, and fragmentation. Including the downwards motion of convective cells for the transport of pebbles with an initial radius of 1 mm, we find pebble sizes between 100 μm and 1 mm near the surface of the protoplanet. These sizes are generally amenable to accretion at the base of the convection flow. Small protoplanets far from the star (>30 au) nevertheless erode their pebbles to sizes below 10 μm; future hydrodynamical simulations will be needed to determine whether such small fragments can detach from the convection flow and become accreted by the protoplanet.
|
|
| 8. |
- Landi, E., et al.
(författare)
-
Hinode/EIS Measurements of Active-region Magnetic Fields
- 2020
-
Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 904:2
-
Tidskriftsartikel (refereegranskat)abstract
- The present work illustrates the potential of a new diagnostic technique that allows the measurement of the coronal magnetic field strength in solar active regions by utilizing a handful of bright Fe x and Fe xi lines commonly observed by the high-resolution Hinode/EUV Imaging Spectrometer (EIS). The importance of this new diagnostic technique is twofold: (1) the coronal magnetic field is probably the most important quantity in coronal physics, being at the heart of the processes regulating space weather and the properties of the solar corona, and (2) this technique can be applied to the existing EIS archive spanning from 2007 to 2020, including more than one full solar cycle and covering a large number of active regions, flares, and even coronal mass ejections. This new diagnostic technique opens the door to a whole new field of studies, complementing the magnetic field measurements from the upcoming DKIST and UCoMP ground-based observatories, and extending our reach to active regions observed on the disk and until now only sampled by radio measurements. In this work, we present a few examples of the application of this technique to EIS observations taken at different times during the EIS mission, and we discuss its current limitations and the steps to improve its accuracy. We also present a list of EIS observing sequences whose data include all of the lines necessary for the application of this diagnostic technique, to help the solar community navigate the immense set of EIS data and to find observations suitable for measuring the coronal magnetic field.
|
|
| 9. |
- Landi, E., et al.
(författare)
-
SUMER measurement of the fex 3p43d 4D5/2,7/2 energy difference
- 2020
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 902:1
-
Tidskriftsartikel (refereegranskat)abstract
- Recent studies have shown that magnetic fields in the solar corona are strong enough to significantly mix the two 3p43d 4D5/2,7/2 levels in Cl-like Fe X. This mixing gives rise to a magnetically induced transition (MIT) component in the bright Fe X 257.3 Å line, commonly observed by current instrumentation, that can be used for coronal magnetic field diagnostics. This line, commonly observed by the still operational EIS spectrometer on board the Hinode satellite since 2007, opens a new window into the coronal magnetic field. However, the strength of this MIT transition depends on the square of the energy difference ΔE of the two 4D5 2,7 2 levels, so that an accurate determination of ΔE is of critical importance to accurately measure coronal magnetic field strengths. In the present work we present a new measurement of ΔE obtained determining the separation of the two component of the Fe X doublet close to 1603.3 Å from deep-exposure spectra of a quiescent streamer at the solar limb taken with the SUMER instrument on board SoHO. Our measurement of ΔE = 2.29 ± 0.50 cm-1 agrees with, and improves upon, an earlier measurements by Judge et al. by decreasing its uncertainty from 80% to approximately 20%, improving the attainable accuracy of magnetic field strength measurements obtainable with the Fe X 257.26 QA line.
|
|
| 10. |
- Nidever, David L., et al.
(författare)
-
The Lazy Giants : APOGEE Abundances Reveal Low Star Formation Efficiencies in the Magellanic Clouds
- 2020
-
Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 895:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report the first APOGEE metallicities and alpha-element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds, the largest Milky Way (MW) dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from [Fe/H] = -0.2 to very metal-poor stars with [Fe/H] -2.5, the most metal-poor Magellanic Cloud (MC) stars detected to date. The LMC [alpha/Fe]-[Fe/H] distribution is very flat over a large metallicity range but rises by similar to 0.1 dex at -1.0 < [Fe/H] less than or similar to -0.5. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent "starburst." At the metal-poor end, we capture the increase of [alpha/Fe] with decreasing [Fe/H] and constrain the "alpha-knee" to [Fe/H] less than or similar to -2.2 in both MCs, implying a low star formation efficiency of similar to 0.01 Gyr(-1). The MC knees are more metal-poor than those of less massive MW dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW's gravitational potential only recently.
|
|
