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Berlin Brandenburg

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  • 1
    Language: English
    In: The Astrophysical Journal, 2019, Vol.882(1), p.18 (15pp)
    Description: Renzini wrote an influential critique of “overshooting” in mixing-length theory (MLT), as used in stellar evolution codes, and concluded that three-dimensional fluid dynamical simulations were needed. Such simulations are now well tested. Implicit large eddy simulations connect large-scale stellar flow to a turbulent cascade at the grid scale, and allow the simulation of turbulent boundary layers, with essentially no assumptions regarding flow except the number of computational cells. Buoyant driving balances turbulent dissipation for weak stratification, as in MLT, but with the dissipation length replacing the mixing length. The turbulent kinetic energy in our computational domain shows steady pulses after 30 turnovers, with no discernible diminution; these are caused by the necessary lag in turbulent dissipation behind acceleration. Interactions between coherent turbulent structures give multi-modal behavior, which drives intermittency and fluctuations. These cause mixing, which may justify use of the instability criterion of Schwarzschild rather than the Ledoux. Chaotic shear flow of turning material at convective boundaries causes instabilities that generate waves and sculpt the composition gradients and boundary layer structures. The flow is not anelastic; wave generation is necessary at boundaries. A self-consistent approach to boundary layers can remove the need for ad hoc procedures of “convective overshooting” and “semi-convection.” In Paper II, we quantify the adequacy of our numerical resolution in a novel way, determine the length scale of dissipation—the “mixing length”—without astronomical calibration, quantify agreement with the four-fifths law of Kolmogorov for weak stratification, and deal with strong stratification.
    Keywords: Astrophysics - Solar And Stellar Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 2
    Language: English
    In: The Astrophysical Journal, 2014, Vol.788(2), p.109 (19pp)
    Description: In tandem with observational data sets, we utilize realistic mock catalogs, based on a semi-analytic galaxy formation model, constructed specifically for Pan-STARRS1 Medium Deep Surveys to assess the performance of the Probability Friends-of-Friends (PFOF) group finder, and aim to develop a grouping optimization method applicable to surveys like Pan-STARRS1. Producing mock PFOF group catalogs under a variety of photometric redshift accuracies ( ), we find that catalog purities and completenesses from good ( 0.01) to poor ( 0.07) photo- z s gradually degrade from 77% and 70% to 52% and 47%, respectively. A subset optimization approach is developed by using spectroscopic-redshift group data from the target field to train the group finder for application to that field and demonstrated using zCOSMOS groups for PFOF searches within PS1 Medium Deep Field04 (PS1MD04) and DEEP2 EGS groups in PS1MD07. With four data sets spanning the photo- z accuracy range from 0.01 to 0.06, we find purities and completenesses agree with their mock analogs. Further tests are performed via matches to X-ray clusters. We find PFOF groups match 85% of X-ray clusters identified in COSMOS and PS1MD04, lending additional support to the reliability of the detection algorithm. In the end, we demonstrate, by separating red and blue group galaxies in the EGS and PS1MD07 group catalogs, that the algorithm is not biased with respect to specifically recovering galaxies by color. The analyses suggest the PFOF algorithm shows great promise as a reliable group finder for photometric galaxy surveys of varying depth and coverage.
    Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 3
    Language: English
    In: The Astrophysical Journal, 2012, Vol.761(2), p.161 (16pp)
    Description: We present a medium-resolution spectroscopic survey of late-type giant stars at mid-Galactic latitudes of (30° 〈 | b | 〈 60°), designed to probe the properties of this population to distances of 9 kpc. Because M giants are generally metal-rich and we have limited contamination from thin disk stars by the latitude selection, most of the stars in the survey are expected to be members of the thick disk ([Fe/H] 0.6) with some contribution from the metal-rich component of the nearby halo. Here we report first results for 1799 stars. The distribution of radial velocity (RV) as a function of l for these stars shows (1) the expected thick disk population and (2) local metal-rich halo stars moving at high speeds relative to the disk, which in some cases form distinct sequences in RV- l space. High-resolution echelle spectra taken for 34 of these RV outliers reveal the following patterns across the [Ti/Fe]-[Fe/H] plane: 17 of the stars have abundances reminiscent of the populations present in dwarf satellites of the Milky Way, 8 have abundances coincident with those of the Galactic disk and a more metal-rich halo, and 9 of the stars fall on the locus defined by the majority of stars in the halo. The chemical abundance trends of the RV outliers suggest that this sample consists predominantly of stars accreted from infalling dwarf galaxies. A smaller fraction of stars in the RV outlier sample may have been formed in the inner Galaxy and subsequently kicked to higher eccentricity orbits, but the sample is not large enough to distinguish conclusively between this interpretation and the alternative that these stars represent the tail of the velocity distribution of the thick disk. Our data do not rule out the possibility that a minority of the sample could have formed from gas in situ on their current orbits. These results are consistent with scenarios where the stellar halo, at least as probed by M giants, arises from multiple formation mechanisms; however, when taken at face value, our results for metal-rich halo giants suggest a much higher proportion to be accreted than found by Carollo et al. and more like the fraction suggested in the analysis by Nissen & Schuster and Schuster et al. We conclude that M giants with large RVs can provide particularly fruitful samples to mine for accreted structures and that some of the velocity sequences may indeed correspond to real physical associations resulting from recent accretion events.
    Keywords: Astrophysics, Cosmology And Astronomy ; Astronomy ; Astrophysics ; Element Abundance ; Emission Spectroscopy ; Giant Stars ; Hydrogen ; Iron ; Milky Way ; Orbits ; Radial Velocity ; Resolution ; Star Accretion ; Titanium ; Astronomy & Astrophysics ; Physics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 4
    In: The Astrophysical Journal, 11/20/2017, Vol.850(1), p.96
    Description: We present a comprehensive and precise description of the Sagittarius (Sgr) stellar stream's 3D geometry as traced by its old stellar population. This analysis draws on the sample of ${\sim}44,000$ RR Lyrae (RRab) stars from the Pan-STARRS1 (PS1) 3$\pi$ survey (Hernitschek et al. 2016,Sesar et al. 2017b), which is ${\sim}80\%$ complete and ${\sim}90\%$ pure within 80~kpc, and extends to ${\gtrsim} 120$~kpc with a distance precision of ${\sim} 3\%$. A projection of RR Lyrae stars within $|\tilde{B}|_{\odot}〈9^\circ$ of the Sgr stream's orbital plane reveals the morphology of both the leading and the trailing arms at very high contrast, across much of the sky. In particular, the map traces the stream near-contiguously through the distant apocenters. We fit a simple model for the mean distance and line-of-sight depth of the Sgr stream as a function of the orbital plane angle $\tilde{\Lambda}_{\odot}$, along with a power-law background-model for the field stars. This modeling results in estimates of the mean stream distance precise to ${\sim}1\%$ and it resolves the stream's line-of-sight depth. These improved geometric constraints can serve as new constraints for dynamical stream models.
    Keywords: Astrophysics - Astrophysics Of Galaxies;
    ISSN: The Astrophysical Journal
    E-ISSN: 1538-4357
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  • 5
    Language: English
    In: The Astrophysical Journal, 2016, Vol.822(2), p.66 (12pp)
    Description: We present a recalibration of the Sloan Digital Sky Survey (SDSS) photometry with new flat fields and zero points derived from Pan-STARRS1. Using point-spread function (PSF) photometry of 60 million stars with 16 〈  r  〈 20, we derive a model of amplifier gain and flat-field corrections with per-run rms residuals of 3 millimagnitudes (mmag) in griz bands and 15 mmag in u band. The new photometric zero points are adjusted to leave the median in the Galactic north unchanged for compatibility with previous SDSS work. We also identify transient non-photometric periods in SDSS (“contrails”) based on photometric deviations co-temporal in SDSS bands. The recalibrated stellar PSF photometry of SDSS and PS1 has an rms difference of {9, 7, 7, 8} mmag in griz , respectively, when averaged over 15′ regions.
    Keywords: Astrophysics - Instrumentation And Methods For Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 6
    Language: English
    In: The Astrophysical Journal, 2016, Vol.817(1), p.73 (26pp)
    Description: In area and depth, the Pan-STARRS1 (PS1) 3 π survey is unique among many-epoch, multi-band surveys and has enormous potential for the all-sky identification of variable sources. PS1 has observed the sky typically seven times in each of its five bands ( grizy ) over 3.5 years, but unlike SDSS, not simultaneously across the bands. Here we develop a new approach for quantifying statistical properties of non-simultaneous, sparse, multi-color light curves through light curve structure functions, effectively turning PS1 into a ∼35-epoch survey. We use this approach to estimate variability amplitudes and timescales ( ω r , τ ) for all point sources brighter than r P1  = 21.5 mag in the survey. With PS1 data on SDSS Stripe 82 as “ground truth,” we use a Random Forest Classifier to identify QSOs and RR Lyrae based on their variability and their mean PS1 and WISE colors. We find that, aside from the Galactic plane, QSO and RR Lyrae samples of purity ∼75% and completeness ∼92% can be selected. On this basis we have identified a sample of ∼1,000,000 QSO candidates, as well as an unprecedentedly large and deep sample of ∼150,000 RR Lyrae candidates with distances from ∼10 to ∼120 kpc. Within the Draco dwarf spheroidal, we demonstrate a distance precision of 6% for RR Lyrae candidates. We provide a catalog of all likely variable point sources and likely QSOs in PS1, a total of 25.8 × 10 6 sources.
    Keywords: Astrophysics - Astrophysics Of Galaxies;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 7
    Language: English
    In: The Astrophysical Journal, 2016, Vol.825(2), p.140 (26pp)
    Description: Using the Pan-STARRS1 survey, we derive limiting magnitude, spatial completeness, and density maps that we use to probe the three-dimensional structure and estimate the stellar mass of the so-called Monoceros Ring. The Monoceros Ring is an enormous and complex stellar sub-structure in the outer Milky Way disk. It is most visible across the large Galactic Anticenter region, , . We estimate its stellar mass density profile along every line of sight in 2° × 2° pixels over the entire 30,000 deg 2 Pan-STARRS1 survey using the previously developed match software. By parsing this distribution into a radially smooth component and the Monoceros Ring, we obtain its mass and distance from the Sun along each relevant line of sight. The Monoceros Ring is significantly closer to us in the south (6 kpc) than in the north (9 kpc). We also create 2D cross-sections parallel to the Galactic plane that show 135° of the Monoceros Ring in the south and 170° of the Monoceros Ring in the north. We show that the northern and southern structures are also roughly concentric circles, suggesting that they may be waves rippling from a common origin. Excluding the Galactic plane , we observe an excess mass of across . If we interpolate across the Galactic plane, we estimate that this region contains . If we assume (somewhat boldly) that the Monoceros Ring is a set of two Galactocentric rings, its total mass is . Finally, if we assume that it is a set of two circles centered at a point 4 kpc from the Galactic center in the anti-central direction, as our data suggests, we estimate its mass to be .
    Keywords: Planes ; Constraining ; Estimates ; Line of Sight ; Density ; Stellar Mass ; Maps ; Cross Sections ; Astrophysics (Ah) ; Galaxy: Disk;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 8
    Language: English
    In: The Astrophysical Journal, 2019, Vol.874(1), p.8 (21pp)
    Description: Active galactic nuclei (AGNs) that show strong rest-frame optical/UV variability in their blue continuum and broad line emission are classified as changing-look AGN, or at higher luminosities, changing-look quasars (CLQs). These surprisingly large and sometimes rapid transitions challenge accepted models of quasar physics and duty cycles, offer several new avenues for study of quasar host galaxies, and open a wider interpretation of the cause of differences between broad and narrow-line AGN. To better characterize extreme quasar variability, we present follow-up spectroscopy as part of a comprehensive search for CLQs across the full Sloan Digital Sky Survey (SDSS) footprint using spectroscopically confirmed quasars from the SDSS DR7 catalog. Our primary selection requires large-amplitude ( mag, mag) variability over any of the available time baselines probed by the SDSS and Pan-STARRS 1 surveys. We employ photometry from the Catalina Sky Survey to verify variability behavior in CLQ candidates where available, and confirm CLQs using optical spectroscopy from the William Herschel, MMT, Magellan, and Palomar telescopes. For our adopted signal-to-noise ratio threshold on variability of broad H β emission, we find 17 new CLQs, yielding a confirmation rate of ≳20%. These candidates are at lower Eddington ratio relative to the overall quasar population, which supports a disk-wind model for the broad line region. Based on our sample, the CLQ fraction increases from 10% to roughly half as the continuum flux ratio between repeat spectra at 3420 Å increases from 1.5 to 6. We release a catalog of more than 200 highly variable candidates to facilitate future CLQ searches.
    Keywords: Astrophysics - Astrophysics Of Galaxies;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 9
    Language: English
    In: The Astrophysical Journal Letters, 2013, Vol.771(1), p.L1 (6pp)
    Description: We present initial results from observations and numerical analyses aimed at characterizing the main-belt comet P/2012 T1 (PANSTARRS). Optical monitoring observations were made between 2012 October and 2013 February using the University of Hawaii 2.2 m telescope, the Keck I telescope, the Baade and Clay Magellan telescopes, Faulkes Telescope South, the Perkins Telescope at Lowell Observatory, and the Southern Astrophysical Research Telescope. The object's intrinsic brightness approximately doubles from the time of its discovery in early October until mid-November and then decreases by 60% between late December and early February, similar to photometric behavior exhibited by several other main-belt comets and unlike that exhibited by disrupted asteroid (596) Scheila. We also used Keck to conduct spectroscopic searches for CN emission as well as absorption at 0.7 m that could indicate the presence of hydrated minerals, finding an upper limit CN production rate of Q CN 〈 1.5 × 10 23  mol s 1 , from which we infer a water production rate of  mol s 1 , and no evidence of the presence of hydrated minerals. Numerical simulations indicate that P/2012 T1 is largely dynamically stable for 〉100 Myr and is unlikely to be a recently implanted interloper from the outer solar system, while a search for potential asteroid family associations reveals that it is dynamically linked to the 155 Myr old Lixiaohua asteroid family.
    Keywords: Extraterrestrial Geology ; Asteroids ; Brightness ; Comets ; Dynamics ; Eccentricity ; Hydrates ; Lixiaohua Asteroid Family ; Main-Belt Comets ; Monitoring ; Numerical Analysis ; Numerical Models ; Optical Spectra ; Orbits ; P/2012 T1 Comet ; Pan-Starrs ; Panoramic Survey Telescope And Rapid Response System ; Photometry ; Reflectance ; Simulation ; Spectra ; Stability ; Sublimation ; Surveys ; Telescope Methods ; Water;
    ISSN: 2041-8205
    E-ISSN: 2041-8213
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  • 10
    Language: English
    In: The Astrophysical Journal Letters, 2018, Vol.855(1), p.L6 (9pp)
    Description: We report the discovery of an H r  = 3.4 ± 0.1 dwarf planet candidate by the Pan-STARRS Outer Solar System Survey. 2010 JO 179 is red with ( g − r ) = 0.88 ± 0.21, roughly round, and slowly rotating, with a period of 30.6 hr. Estimates of its albedo imply a diameter of 600–900 km. Observations sampling the span between 2005 and 2016 provide an exceptionally well determined orbit for 2010 JO 179 , with a semimajor axis of 78.307 ± 0.009 au; distant orbits known to this precision are rare. We find that 2010 JO 179 librates securely within the 21:5 mean-motion resonance with Neptune on 100 Myr timescales, joining the small but growing set of known distant dwarf planets on metastable resonant orbits. These imply a substantial trans-Neptunian population that shifts between stability in high-order resonances, the detached population, and the eroding population of the scattering disk.
    Keywords: Astrophysics - Earth And Planetary Astrophysics;
    ISSN: 2041-8205
    E-ISSN: 2041-8213
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