| 1261. |
- Norris, J. E., et al.
(författare)
-
The most metal-poor stars. I. Discovery, data, and atmospheric parameters
- 2013
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 762:1, s. 25-
-
Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of 34 stars in the Hamburg/ESO Survey for metal-poor stars and the Sloan Digital Sky Survey that have [Fe/H] ≲ -3.0. Their median and minimum abundances are [Fe/H] = -3.1 and -4.1, respectively, while 10 stars have [Fe/H] < -3.5. High-resolution, high signal-to-noise spectroscopic data - equivalent widths and radial velocities - are presented for these stars, together with an additional four objects previously reported or currently being investigated elsewhere. We have determined the atmospheric parameters, effective temperature (T eff), and surface gravity (log g), which are critical in the determination of the chemical abundances and the evolutionary status of these stars. Three techniques were used to derive these parameters. Spectrophotometric fits to model atmosphere fluxes were used to derive T eff, log g, and an estimate of E(B - V); Hα, Hβ, and Hγ profile fitting to model atmosphere results provided the second determination of T eff and log g; and finally, we used an empirical T eff-calibrated Hδ index, for the third, independent T eff determination. The three values of T eff are in good agreement, although the profile fitting may yield systematically cooler T eff values, by ∼100 K. This collective data set will be analyzed in future papers in the present series to utilize the most metal-poor stars as probes of conditions in the early universe.
|
|
| 1262. |
- Norris, J. E., et al.
(författare)
-
The most metal-poor stars. IV. the two populations with [Fe/H] ≲ -3.0
- 2013
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 762:1, s. 28-
-
Tidskriftsartikel (refereegranskat)abstract
- We discuss the carbon-normal and carbon-rich populations of Galactic halo stars having [Fe/H] ≲ -3.0, utilizing chemical abundances from high-resolution, high signal-to-noise model-atmosphere analyses. The C-rich population represents ∼28% of stars below [Fe/H] = -3.1, with the present C-rich sample comprising 16 CEMP-no stars, and two others with [Fe/H] ∼ -5.5 and uncertain classification. The population is O-rich ([O/Fe] ≳ +1.5); the light elements Na, Mg, and Al are enhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is little evidence for enhancements relative to solar values. These results are best explained in terms of the admixing and processing of material from H-burning and He-burning regions as achieved by nucleosynthesis in zero-heavy-element models in the literature of "mixing and fallback" supernovae (SNe); of rotating, massive, and intermediate-mass stars; and of Type II SNe with relativistic jets. The available (limited) radial velocities offer little support for the C-rich stars with [Fe/H] < -3.1 being binary. More data are required before one could conclude that binarity is key to an understanding of this population. We suggest that the C-rich and C-normal populations result from two different gas cooling channels in the very early universe of material that formed the progenitors of the two populations. The first was cooling by fine-structure line transitions of C II and O I (to form the C-rich population); the second, while not well defined (perhaps dust-induced cooling?), led to the C-normal group. In this scenario, the C-rich population contains the oldest stars currently observed.
|
|
| 1263. |
- Norris, John E., et al.
(författare)
-
The oxygen abundance of the ultra-metal-poor star HE 0557-4840
- 2012
-
Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 753:2, s. 150-
-
Tidskriftsartikel (refereegranskat)abstract
- We present a high-resolution ultraviolet (UV) spectrum of the ultra-metal-poor (UMP) carbon-enhanced red giant HE 0557-4840 (T-eff/log g/[Fe/H] = 4900/2.2/-4.8). Combining these data with earlier observations, the radial velocity is 212.0 +/- 0.4 km s(-1), with no evidence of variability during 2006 February to 2007 December. One-dimensional (1D) LTE model-atmosphere analysis of UV Fe and CH lines confirms the iron and carbon abundances obtained previously ([Fe/H] = -4.8 and [C/Fe](1D) = +1.7), and places a more stringent limit on nitrogen abundance of [N/Fe](1D) < +1.0. Analysis of the UV OH lines yields [O/Fe](1D) = +2.3 +/- 0.4. When corrections are made for three-dimensional (3D) effects we obtain [C/Fe](3D) = +1.1, [N/Fe](3D) < +0.1, and [O/Fe](3D) = +1.4. Comparison of the abundances of HE 0557-4840 with those of supernova models of Nomoto et al. and Joggerst et al. suggests that none is able to explain fully the observed abundance pattern. For HE 0557-4840, the Frebel et al. transition discriminant D-trans(= log(10([C/H]) + 0.3 x 10([O/H])) = -3.4 +/- 0.2, consistent with fine-structure transitions of C II and O I being a major cooling mechanism of star-forming regions at the earliest times. Of the four stars known to have [Fe/H] less than or similar to -4.3, three are strongly carbon and oxygen enhanced. If the suggestion by Caffau et al. that SDSS J102915+172927 ([Fe/H] = -4.7) does not belong to the class of C-rich, O-rich, UMP stars is supported by future similar discoveries, one will need to consider multiple channels for the production of stars having [Fe/H] less than or similar to -4.3.
|
|
| 1264. |
- Noutsos, A., et al.
(författare)
-
Radio and γ-ray constraints on the emission geometry and birthplace of PSR j2043+2740
- 2011
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 728:2, s. 77-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the first year of Fermi γ-ray observations of pulsed high-energy emission from the old PSR J2043 + 2740. The study of the γ-ray efficiency of such old pulsars gives us an insight into the evolution of pulsars' ability to emit in γ rays as they age. The y-ray light curve of this pulsar above 0.1 GeV is clearly defined by two sharp peaks, 0.353 ± 0.035 periods apart. We have combined the γ-ray profile characteristics of PSR J2043 + 2740 with the geometrical properties of the pulsar's radio emission, derived from radio-polarization data, and constrained the pulsar-beam geometry in the framework of a two-pole caustic (TPC) and an outer gap (OG) model. The ranges of magnetic inclination and viewing angle were determined to be {α, ζ} ∼ {52°-57°, 61°-68°} for the TPC model, and {α, ζ} ∼ {62°-73°, 74°-81°} and {α, ζ,} ∼ {72°-83°, 60°-75°} for the OG model. Based on this geometry, we assess possible birth locations for this pulsar and derive a likely proper motion, sufficiently high to be measurable with VLBI. At a characteristic age of 1.2 Myr, PSR J2043 + 2740 is the third oldest of all discovered, non-recycled, γ-ray pulsars: it is twice as old as the next oldest, PSR J0357 + 32, and younger only than the recently discovered PSR J1836 + 5925 and PSR J2055 + 25, both of which are at least five and ten times less energetic, respectively.
|
|
| 1265. |
- Novotny, O., et al.
(författare)
-
DISSOCIATIVE RECOMBINATION MEASUREMENTS OF HCl+ USING AN ION STORAGE RING
- 2013
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 777:1, s. 54-
-
Tidskriftsartikel (refereegranskat)abstract
- We have measured dissociative recombination (DR) of HCl+ with electrons using a merged beams configuration at the TSR heavy-ion storage ring located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We present the measured absolute merged beams recombination rate coefficient for collision energies from 0 to 4.5 eV. We have also developed a new method for deriving the cross section from the measurements. Our approach does not suffer from approximations made by previously used methods. The cross section was transformed to a plasma rate coefficient for the electron temperature range from T = 10 to 5000 K. We show that the previously used HCl+ DR data underestimate the plasma rate coefficient by a factor of 1.5 at T = 10 K and overestimate it by a factor of three at T = 300 K. We also find that the new data may partly explain existing discrepancies between observed abundances of chlorine-bearing molecules and their astrochemical models.
|
|
| 1266. |
- Novotný, O., et al.
(författare)
-
DISSOCIATIVE RECOMBINATION MEASUREMENTS OF NH+ USING AN ION STORAGE RING
- 2014
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 792:2
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated dissociative recombination (DR) of NH+ with electrons using a merged beams configuration at the TSR heavy-ion storage ring located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We present our measured absolute merged-beams recombination rate coefficient for collision energies from 0 to 12 eV. From these data, we have extracted a cross section, which we have transformed to a plasma rate coefficient for the collisional plasma temperature range from T-p1 = 10 to 18,000 K. We show that the NH+ DR rate coefficient data in current astrochemical models are underestimated by up to a factor of approximately nine. Our new data will result in predicted NH+ abundances lower than those calculated by present models. This is in agreement with the sensitivity limits of all observations attempting to detect NH+ in interstellar clouds.
|
|
| 1267. |
- Ofek, Eran O., et al.
(författare)
-
INTERACTION-POWERED SUPERNOVAE : RISE-TIME VERSUS PEAK-LUMINOSITY CORRELATION AND THE SHOCK-BREAKOUT VELOCITY
- 2014
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 788:2, s. 154-
-
Tidskriftsartikel (refereegranskat)abstract
- Interaction of supernova (SN) ejecta with the optically thick circumstellar medium (CSM) of a progenitor star can result in a bright, long-lived shock-breakout event. Candidates for such SNe include Type IIn and superluminous SNe. If some of these SNe are powered by interaction, then there should be a specific relation between their peak luminosity, bolometric light-curve rise time, and shock-breakout velocity. Given that the shock velocity during shock breakout is not measured, we expect a correlation, with a significant spread, between the rise time and the peak luminosity of these SNe. Here, we present a sample of 15 SNe IIn for which we have good constraints on their rise time and peak luminosity from observations obtained using the Palomar Transient Factory. We report on a possible correlation between the R-band rise time and peak luminosity of these SNe, with a false-alarm probability of 3%. Assuming that these SNe are powered by interaction, combining these observables and theory allows us to deduce lower limits on the shock-breakout velocity. The lower limits on the shock velocity we find are consistent with what is expected for SNe (i.e., similar to 10(4) km s(-1)). This supports the suggestion that the early-time light curves of SNe IIn are caused by shock breakout in a dense CSM. We note that such a correlation can arise from other physical mechanisms. Performing such a test on other classes of SNe ( e. g., superluminous SNe) can be used to rule out the interaction model for a class of events.
|
|
| 1268. |
- Ofek, E. O., et al.
(författare)
-
PTF13efv-AN OUTBURST 500 DAYS PRIOR TO THE SNHUNT 275 EXPLOSION AND ITS RADIATIVE EFFICIENCY
- 2016
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 824:1
-
Tidskriftsartikel (refereegranskat)abstract
- The progenitors of some supernovae (SNe) exhibit outbursts with super-Eddington luminosities prior to their final explosions. This behavior is common among SNe IIn, but the driving mechanisms of these precursors are not yet well-understood. SNHunt 275 was announced as a possible new SN during 2015 May. Here we report on pre-explosion observations of the location of this event by the Palomar Transient Factory (PTF) and report the detection of a precursor about 500 days prior to the 2015 May activity (PTF 13efv). The observed velocities in the 2015 transient and its 2013 precursor absorption spectra are low (1000-2000 km s(-1)), so it is not clear yet if the recent activity indeed marks the final disruption of the progenitor. Regardless of the nature of this event, we use the PTF photometric and spectral observations, as well as Swift-UVOT observations, to constrain the efficiency of the radiated energy relative to the total kinetic energy of the precursor. We find that, using an order-of-magnitude estimate and under the assumption of spherical symmetry, the ratio of the radiated energy to the kinetic energy is in the range of 4 x 10(-2) to 3.4 x 10(3).
|
|
| 1269. |
- Olshevsky, Vyacheslav, et al.
(författare)
-
ENERGY DISSIPATION IN MAGNETIC NULL POINTS AT KINETIC SCALES
- 2015
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 807:2
-
Tidskriftsartikel (refereegranskat)abstract
- We use kinetic particle-in-cell and MHD simulations supported by an observational data set to investigate magnetic reconnection in clusters of null points in space plasma. The magnetic configuration under investigation is driven by fast adiabatic flux rope compression that dissipates almost half of the initial magnetic field energy. In this phase powerful currents are excited producing secondary instabilities, and the system is brought into a state of "intermittent turbulence" within a few ion gyro-periods. Reconnection events are distributed all over the simulation domain and energy dissipation is rather volume-filling. Numerous spiral null points interconnected via their spines form null lines embedded into magnetic flux ropes; null point pairs demonstrate the signatures of torsional spine reconnection. However, energy dissipation mainly happens in the shear layers formed by adjacent flux ropes with oppositely directed currents. In these regions radial null pairs are spontaneously emerging and vanishing, associated with electron streams and small-scale current sheets. The number of spiral nulls in the simulation outweighs the number of radial nulls by a factor of 5-10, in accordance with Cluster observations in the Earth's magnetosheath. Twisted magnetic fields with embedded spiral null points might indicate the regions of major energy dissipation for future space missions such as the Magnetospheric Multiscale Mission.
|
|
| 1270. |
- Olshevsky, Vyacheslav, et al.
(författare)
-
Magnetic Null Points In Kinetic Simulations of Space Plasmas
- 2016
-
Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 819:1
-
Tidskriftsartikel (refereegranskat)abstract
- We present a systematic attempt to study magnetic null points and the associated magnetic energy conversion in kinetic particle-in-cell simulations of various plasma configurations. We address three-dimensional simulations performed with the semi-implicit kinetic electromagnetic code iPic3D in different setups: variations of a Harris current sheet, dipolar and quadrupolar magnetospheres interacting with the solar wind,. and a relaxing turbulent configuration with multiple null points. Spiral nulls are more likely created in space plasmas: in all our simulations except lunar magnetic anomaly (LMA) and quadrupolar mini-magnetosphere the number of spiral nulls prevails over the number of radial nulls by a factor of 3-9. We show that often magnetic nulls do not indicate the regions of intensive energy dissipation. Energy dissipation events caused by topological bifurcations at radial nulls are rather rare and short-lived. The so-called X-lines formed by the radial nulls in the Harris current sheet and LMA simulations are rather stable and do not exhibit any energy dissipation. Energy dissipation is more powerful in the vicinity of spiral nulls enclosed by magnetic flux ropes with strong currents at their axes (their cross. sections resemble 2D magnetic islands). These null lines reminiscent of Z-pinches efficiently dissipate magnetic energy due to secondary instabilities such as the two-stream or kinking instability, accompanied by changes in magnetic topology. Current enhancements accompanied by spiral nulls may signal magnetic energy conversion sites in the observational data.
|
|
