| 1. |
- Church, Ross P., et al.
(author)
-
Formation Constraints Indicate a Black Hole Accretor in 47 Tuc X9
- 2017
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 851:1
-
Journal article (peer-reviewed)abstract
- The luminous X-ray binary 47 Tuc X9 shows radio and X-ray emission consistent with a stellar-mass black hole (BH) accreting from a carbon-oxygen white dwarf. Its location, in the core of the massive globular cluster 47 Tuc, hints at a dynamical origin. We assess the stability of mass transfer from a carbon-oxygen white dwarf onto compact objects of various masses, and conclude that for mass transfer to proceed stably, the accretor must, in fact, be a BH. Such systems can form dynamically by the collision of a stellar-mass BH with a giant star. Tidal dissipation of energy in the giant's envelope leads to a bound binary with a pericenter separation less than the radius of the giant. An episode of common-envelope evolution follows, which ejects the giant's envelope. We find that the most likely target is a horizontal-branch star, and that a realistic quantity of subsequent dynamical hardening is required for the resulting binary to merge via gravitational wave emission. Observing one binary like 47 Tuc X9 in the Milky Way globular cluster system is consistent with the expected formation rate. The observed 6.8-day periodicity in the X-ray emission may be driven by eccentricity induced in the ultra-compact X-ray binary's orbit by a perturbing companion.
|
|
| 2. |
- Henkel, Kate, et al.
(author)
-
Thermohaline Mixing in Extremely Metal-poor Stars
- 2018
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 863:1
-
Journal article (peer-reviewed)abstract
- Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well studied, they make up less than half of all EMP stars with [Fe/H] ∼ -3.0. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offsets are derived) is very important; however, the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coefficient. Our stellar evolution models utilize a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at [Fe/H] = -3 and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.
|
|
| 3. |
- Izumi, T., et al.
(author)
-
On the Disappearance of a Cold Molecular Torus around the Low-luminosity Active Galactic Nucleus of NGC 1097
- 2017
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 845:1, s. L5-
-
Journal article (peer-reviewed)abstract
- We used the Atacama Large Millimeter/Submillimeter Array to map the CO(3-2) and the underlying continuum emissions around the type-1low-luminosity active galactic nucleus (LLAGN; bolometric luminosity less than or similar to 10(42) erg. s(-1)) of NGC 1097 at similar to 10 pc resolution. These observations revealed a detailed cold gas distribution within a similar to 100 pc of this LLAGN. In contrast to the luminous Seyfert galaxy NGC 1068, where a similar to 7 pc cold molecular torus was recently revealed, a distinctively dense and compact torus is missing in our CO(3-2) integrated intensity map of NGC 1097. Based on the CO(3-2) flux, the gas mass of the torus of NGC 1097 would be a factor of greater than or similar to 2-3 less than that found for NGC 1068 by using the same CO-to-H-2 conversion factor, which implies less active nuclear star formation and/or inflows in NGC 1097. Our dynamical modeling of the CO(3-2) velocity field implies that the cold molecular gas is concentrated in a thin layer as compared to the hot gas traced by the 2.12 mu m H-2 emission in and around the torus. Furthermore, we suggest that NGC 1097 hosts a geometrically thinner torus than NGC 1068. Although the physical origin of the torus thickness remains unclear, our observations support a theoretical prediction that geometrically thick tori with high opacity will become deficient as AGNs evolve from luminous Seyferts to LLAGNs.
|
|
| 4. |
- Sahlholdt, Christian L., et al.
(author)
-
Characteristics of the Two Sequences Seen in the High-velocity Hertzsprung-Russell Diagram in Gaia DR2
- 2019
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 881:1
-
Journal article (peer-reviewed)abstract
- In this study we use a sample of about 9 million SkyMapper stars with metallicities to investigate the properties of the two stellar populations seen in the high-velocity (V T > 200 km s-1) Gaia DR2 Hertzsprung-Russell diagram. Based on 10,000 red giant branch (RGB) stars (out of 75,000 with high velocity), we find that the two sequences have different metallicity distribution functions; one peaks at -1.4 dex (blue sequence) and the other at -0.7 dex (red sequence). Isochrones with ages in the range 11-13.5 Gyr, and metallicities chosen to match the observations for each sequence, fit the turnoffs and broad RGBs well, indicating that the two populations formed at comparable times within the uncertainties. We find that the mean tangential velocity of disk stars increases steadily with decreasing metallicity, and that the red sequence is made up of the high-velocity stars at the lowest metallicities of the thick-disk population. Using relative number densities, we further find that the red-sequence stars are more centrally concentrated in the Galaxy, and we estimate the radial scale length of this population to be on the order of 2-3 kpc. The blue-sequence stars, on the other hand, follow a nearly flat radial density profile. These findings tighten the link between the red-sequence stars and the chemically defined thick disk.
|
|
| 5. |
- Souto, Diogo, et al.
(author)
-
Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra
- 2018
-
In: Astrophysical Journal Letters. - : Institute of Physics Publishing (IOPP). - 2041-8205 .- 2041-8213. ; 860:1
-
Journal article (peer-reviewed)abstract
- The first detailed chemical abundance analysis of the M-dwarf (M4.0) exoplanet-hosting star Ross 128 is presented here, based upon near-infrared (1.5-1.7 mu m), high-resolution (R similar to 22,500) spectra from the SDSS Apache Point Galactic Evolution Experiment survey. We determined precise atmospheric parameters T-eff = 3231 +/- 100 K, log g = 4.96 +/- 0.11 dex and chemical abundances of eight elements (C, O, Mg, Al, K, Ca, Ti, and Fe), finding Ross 128 to have near solar metallicity ([Fe/H] = +0.03 +/- 0.09 dex). The derived results were obtained via spectral synthesis (1D LTE) adopting both MARCS and PHOENIX model atmospheres; stellar parameters and chemical abundances derived from the different adopted models do not show significant offsets. Mass-radius modeling of Ross 128b indicates that it lies below the pure-rock composition curve, suggesting that it contains a mixture of rock and iron, with the relative amounts of each set by the ratio of Fe/Mg. If Ross 128b formed with a subsolar Si abundance, and assuming the planet's composition matches that of the host star, it likely has a larger core size relative to the Earth despite this producing a planet with a Si/Mg abundance ratio similar to 34% greater than the Sun. The derived planetary parameters-insolation flux (S-Earth = 1.79 +/- 0.26) and equilibrium temperature (T-eq = 294. +/-. 10 K)-support previous findings that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.
|
|
| 6. |
- Tomičić, Neven, et al.
(author)
-
Two Orders of Magnitude Variation in the Star Formation Efficiency across the Premerger Galaxy NGC 2276
- 2018
-
In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 869:2
-
Journal article (peer-reviewed)abstract
- We present the first spatially resolved (∼0.5 kpc) measurements of the molecular gas depletion time τ depl across the disk of the interacting spiral galaxy NGC 2276, a system with an asymmetric morphology in various star formation rate (SFR) tracers. To estimate τ depl, we use new NOEMA observations of the 12CO(1-0) emission tracing the bulk molecular gas reservoir in NGC 2276, and extinction-corrected Hα measurements obtained with the PMAS/PPaK integral field unit for robust estimates of the SFR. We find a systematic decrease in τ depl of 1-1.5 dex across the disk of NGC 2276, with a further, abrupt drop in τ depl of ∼1 dex along the galaxy's western edge. The global τ depl in NGC 2776 is τdepl = 0.55 Gyr, consistent with literature measurements for the nearby galaxy population. Such a large range in τ depl on subkiloparsec scales has never previously been observed within an individual isolated or premerger system. When using a metallicity-dependent molecular gas conversion factor the variation decreases by 0.5 dex. We attribute the variation in τ depl to the influence of galactic-scale tidal forces and ram pressure on NGC 2276's molecular interstellar medium. Our observations add to the growing body of numerical and observational evidence that galaxy-galaxy interactions significantly modify the molecular gas properties and star-forming activity within galactic disks throughout the interaction, and not just during the final merger phase.
|
|
| 7. |
- Abdalla, H., et al.
(author)
-
TeV Gamma-Ray Observations of the Binary Neutron Star Merger GW170817 with HESS
- 2017
-
In: Astrophysical Journal Letters. - : Institute of Physics Publishing (IOPP). - 2041-8205 .- 2041-8213. ; 850:2
-
Journal article (peer-reviewed)abstract
- We search for high-energy gamma-ray emission from the binary neutron star merger GW170817 with the H.E.S.S. Imaging Air Cherenkov Telescopes. The observations presented here have been obtained starting only 5.3 hr after GW170817. The H.E.S.S. target selection identified regions of high probability to find a counterpart of the gravitational-wave event. The first of these regions contained the counterpart SSS17a that has been identified in the optical range several hours after our observations. We can therefore present the first data obtained by a ground-based pointing instrument on this object. A subsequent monitoring campaign with the H.E.S.S. telescopes extended over several days, covering timescales from 0.22 to 5.2 days and energy ranges between 270 GeV to 8.55 TeV. No significant gamma-ray emission has been found. The derived upper limits on the very-high-energy gamma-ray flux for the first time constrain non-thermal, high-energy emission following the merger of a confirmed binary neutron star system.
|
|
| 8. |
- Ajello, M., et al.
(author)
-
Bright Gamma-Ray Flares Observed in GRB 131108A
- 2019
-
In: Astrophysical Journal Letters. - : Institute of Physics (IOP). - 2041-8205 .- 2041-8213. ; 886:2
-
Journal article (peer-reviewed)abstract
- GRB 131108A is a bright long gamma-ray burst (GRB) detected by the Large Area Telescope and the Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope. Dedicated temporal and spectral analyses reveal three ?-ray flares dominating above 100 MeV, which are not directly related to the prompt emission in the Gamma-ray Burst Monitor band (10 keV?10 MeV). The high-energy light curve of GRB 131108A (100 MeV?10 GeV) shows an unusual evolution: a steep decay, followed by three flares with an underlying emission, and then a long-lasting decay phase. The detailed analysis of the ?-ray flares finds that the three flares are 6?20 times brighter than the underlying emission and are similar to each other. The fluence of each flare, (1.6?2.0)10(?6) erg cm(?2), is comparable to that of emission during the steep decay phase, 1.710(?6) erg cm(?2). The total fluence from three ?-ray flares is 5.310(?6) erg cm(?2). The three ?-ray flares show properties similar to the usual X-ray flares that are sharp flux increases, occurring in ?50% of afterglows, in some cases well after the prompt emission. Also, the temporal and spectral indices during the early steep decay phase and the decaying phase of each flare show the consistency with a relation of the curvature effect (
|
|
| 9. |
- Brandenburg, Axel, et al.
(author)
-
Compensating Faraday Depolarization by Magnetic Helicity in the Solar Corona
- 2017
-
In: Astrophysical Journal Letters. - : IOP PUBLISHING LTD. - 2041-8205 .- 2041-8213. ; 845:2
-
Journal article (peer-reviewed)abstract
- A turbulent dynamo in spherical geometry with an outer corona is simulated to study the sign of magnetic helicity in the outer parts. In agreement with earlier studies, the sign in the outer corona is found to be opposite to that inside the dynamo. Line-of-sight observations of polarized emission are synthesized to explore the feasibility of using the local reduction of Faraday depolarization to infer the sign of helicity of magnetic fields in the solar corona. This approach was previously identified as an observational diagnostic in the context of galactic magnetic fields. Based on our simulations, we show that this method can be successful in the solar context if sufficient statistics are gathered by using averages over ring segments in the corona separately for the regions north and south of the solar equator.
|
|
| 10. |
- Chasapis, A., et al.
(author)
-
In Situ Observation of Intermittent Dissipation at Kinetic Scales in the Earth's Magnetosheath
- 2018
-
In: Astrophysical Journal Letters. - : Institute of Physics Publishing. - 2041-8205 .- 2041-8213. ; 856:1
-
Journal article (peer-reviewed)abstract
- We present a study of signatures of energy dissipation at kinetic scales in plasma turbulence based on observations by the Magnetospheric Multiscale mission (MMS) in the Earth's magnetosheath. Using several intervals, and taking advantage of the high-resolution instrumentation on board MMS, we compute and discuss several statistical measures of coherent structures and heating associated with electrons, at previously unattainable scales in space and time. We use the multi-spacecraft Partial Variance of Increments (PVI) technique to study the intermittent structure of the magnetic field. Furthermore, we examine a measure of dissipation and its behavior with respect to the PVI as well as the current density. Additionally, we analyze the evolution of the anisotropic electron temperature and non-Maxwellian features of the particle distribution function. From these diagnostics emerges strong statistical evidence that electrons are preferentially heated in subproton-scale regions of strong electric current density, and this heating is preferentially in the parallel direction relative to the local magnetic field. Accordingly, the conversion of magnetic energy into electron kinetic energy occurs more strongly in regions of stronger current density, a finding consistent with several kinetic plasma simulation studies and hinted at by prior studies using lower resolution Cluster observations.
|
|
