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Search Results: 1 - 10 of 131970 matches for " Frederic V. Hessman "
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The difficulty of measuring the local dark matter density
Frederic V. Hessman
Physics , 2015, DOI: 10.1051/0004-6361/201526022
Abstract: The analysis of the vertical velocity dispersion of disc stars is the most direct astronomical means of estimating the local dark matter density, $\rho_{DM}$. Current estimates based on the mid-plane dynamic density use a local baryonic correction that ignores the non-local effects of spiral structure and significantly underestimates the amount of dynamically relevant gas; the additional gas plus the remaining uncertainties make it practically impossible to measure $\rho_{DM}$ from mid-plane kinematics alone. The sampling of inhomogeneous tracer populations with different scale-heights and scale-lengths results in a systematic increase in the observed dispersion gradients and changes in the nominal density distributions that, if not properly considered, can be misinterpreted as a sign of more dark matter. If the disc gravity is modelled using an infinite disc, the local variation in the vertical gravity due to the globally exponential disc components results in an underestimation of the baryonic contribution by as much as ~40% Given only the assumptions of stationarity, an axially and vertically symmetric disc, doubly exponential tracer and mass-component density profiles, a phenomenologically justified model for the cross-dispersion component $\sigma_{Rz}$, and a realistic model for $g_z$, it is possible to solve the full vertical Jeans equation analytically for the vertical dispersion $\sigma_{z}(z)$ and hence test the robustness of previous attempts at measuring $\rho_{DM}$. When the model parameters are estimated from SEGUE G dwarf star data, it is still not possible to explain the difference in behaviour seen in the simple thick- and thin-disc datasets reported by Buedenbender et al.. Rather than being a fundamental problem with the kinematical model, this effect appears to be a further sign of the difficulty of defining and handling kinematically homogeneous tracer populations.
The Bosma effect revisited - I. HI and stellar disc scaling models
Frederic V. Hessman,Monika Ziebart
Physics , 2011, DOI: 10.1051/0004-6361/201117199
Abstract: The observed proportionality between the centripetal contribution of the dynamically insignificant HI gas in the discs of spiral galaxies and the dominant contribution of DM - the "Bosma effect" - has been repeatedly mentioned in the literature but largely ignored. We have re-examined the evidence for the Bosma effect by fitting Bosma effect models for 17 galaxies in the THINGS data set, either by scaling the contribution of the HI gas alone or by using both the observed stellar disc and HI gas as proxies. The results are compared with two models for exotic cold DM: internally consistent cosmological NFW models with constrained compactness parameters, and URC models using fully unconstrained Burkert density profiles. The Bosma models that use the stellar discs as additional proxies are statistically nearly as good as the URC models and clearly better than the NFW ones. We thus confirm the correlation between the centripetal effects of DM and that of the interstellar medium of spiral galaxies. The edificacy of "maximal disc" models is explained as the natural consequence of "classic" Bosma models which include the stellar disc as a proxy in regions of reduced atomic gas. The standard explanation - that the effect reflects a statistical correlation between the visible and exotic DM - seems highly unlikely, given that the geometric forms and hence centripetal signatures of spherical halo and disc components are so different. A literal interpretation of the Bosma effect as being due to the presence of significant amounts of disc DM requires a median visible baryon to disc DM ratio of about 40%.
The quest for companions to post-common envelope binaries IV: The 2:1 mean-motion resonance of the planets orbiting NN Serpentis
Klaus Beuermann,Stefan Dreizler,Frederic V. Hessman
Physics , 2013, DOI: 10.1051/0004-6361/201220510
Abstract: We present 69 new mid-eclipse times of the young post-common envelope binary (PCEB) NN Ser, which was previously suggested to possess two circumbinary planets. We have interpreted the observed eclipse-time variations in terms of the light-travel time effect caused by two planets, exhaustively covering the multi-dimensional parameter space by fits in the two binary and ten orbital parameters. We supplemented the fits by stability calculations for all models with an acceptable chi-square. An island of secularly stable 2:1 resonant solutions exists, which coincides with the global chi-square minimum. Our best-fit stable solution yields current orbital periods P_o = 15.47 yr and P_i = 7.65 yr and eccentricities e_o = 0.14 and e_i = 0.22 for the outer (o) and inner (i) planets, respectively. The companions qualify as giant planets, with masses of 7.0 M_Jup and 1.7 M_Jup for the case of orbits coplanar with that of the binary. The two-planet model that starts from the present system parameters has a lifetime greater than 10^8 yr, which significantly exceeds the age of NN Ser of 10^6 yr as a PCEB. The resonance is characterized by libration of the resonant variable Theta_1 and circulation of omega_i-omega_o, the difference between the arguments of periapse of the two planets. No stable non-resonant solutions were found, and the possibility of a 5:2 resonance suggested previously by us is now excluded at the 99.3% confidence level.
The Patchy Accretion Disc in HT Cassiopeiae
Sonja Vrielmann,Frederic V. Hessman,Keith Horne
Physics , 2001, DOI: 10.1046/j.1365-8711.2002.05284.x
Abstract: We have reconstructed the temperatures and surface densities in the quiescent accretion disc in HT Cas by performing a "Physical Parameter Eclipse Mapping" analysis of archival UBVR observations. Using a simple hydrogen slab model and demanding a smooth, maximally artefact-free reconstruction, we derive a formal distance to HT Cas of 207 +- 10 pc, significantly larger than the 133 +- 14 pc we derive from a re-analysis of the data in the literature. We argue that the larger derived distance is probably incorrect but is not produced by inaccuracies in our spectral model or optimization method. The discrepancy can be resolved if the emission regions on the disc are patchy with a filling factor of about 40% of the disc's surface. This solves the problem with the high effective temperatures in the disc -- reducing them to around 6 500 K within a radius of 0.2 R_L1 -- and reduces the derived temperature of the white dwarf and/or boundary layer from 22600 to 15500 K. We discuss several possible sources of the chromospheric emission and its patchiness, including irradiation of the disc, thermal instabilities, spiral-wave-like global structures, and magnetically active regions associated with dynamo action and/or Balbus-Hawley instabilities.
Reengineering observatory operations for the time domain
Robert L. Seaman,W. Thomas Vestrand,Frederic V. Hessman
Physics , 2014, DOI: 10.1117/12.2060918
Abstract: Observatories are complex scientific and technical institutions serving diverse users and purposes. Their telescopes, instruments, software, and human resources engage in interwoven workflows over a broad range of timescales. These workflows have been tuned to be responsive to concepts of observatory operations that were applicable when various assets were commissioned, years or decades in the past. The astronomical community is entering an era of rapid change increasingly characterized by large time domain surveys, robotic telescopes and automated infrastructures, and - most significantly - of operating modes and scientific consortia that span our individual facilities, joining them into complex network entities. Observatories must adapt and numerous initiatives are in progress that focus on redesigning individual components out of the astronomical toolkit. New instrumentation is both more capable and more complex than ever, and even simple instruments may have powerful observation scripting capabilities. Remote and queue observing modes are now widespread. Data archives are becoming ubiquitous. Virtual observatory standards and protocols and astroinformatics data-mining techniques layered on these are areas of active development. Indeed, new large-aperture ground-based telescopes may be as expensive as space missions and have similarly formal project management processes and large data management requirements. This piecewise approach is not enough. Whatever challenges of funding or politics facing the national and international astronomical communities it will be more efficient - scientifically as well as in the usual figures of merit of cost, schedule, performance, and risks - to explicitly address the systems engineering of the astronomical community as a whole.
Planet formation in post-common-envelope binaries
Dominik Schleicher,Stefan Dreizler,Marcel V?lschow,Robi Banerjee,Frederic V. Hessman
Physics , 2015, DOI: 10.1002/asna.201412184
Abstract: To understand the evolution of planetary systems, it is important to investigate planets in highly evolved stellar systems, and to explore the implications of their observed properties with respect to potential formation scenarios. Observations suggest the presence of giant planets in post-common-envelope binaries (PCEBs). A particularly well-studied system with planetary masses of 1.7 M_J and 7.0 M_J is NN Ser. We show here that a pure first-generation scenario where the planets form before the common envelope (CE) phase and the orbits evolve due to the changes in the gravitational potential is inconsistent with the current data. We propose a second-generation scenario where the planets are formed from the material that is ejected during the CE, which may naturally explain the observed planetary masses. In addition, hybrid scenarios where the planets form before the CE and evolve due to the accretion of the ejected gas appear as a realistic possibility.
IVOA Recommendation: Vocabularies in the Virtual Observatory Version 1.19
Sebastien Derriere,Alasdair J G Gray,Norman Gray,Frederic V Hessman,Tony Linde,Andrea Preite Martinez,Rob Seaman,Brian Thomas
Physics , 2011,
Abstract: This document specifies a standard format for vocabularies based on the W3C's Resource Description Framework (RDF) and Simple Knowledge Organization System (SKOS). By adopting a standard and simple format, the IVOA will permit different groups to create and maintain their own specialised vocabularies while letting the rest of the astronomical community access, use, and combine them. The use of current, open standards ensures that VO applications will be able to tap into resources of the growing semantic web. The document provides several examples of useful astronomical vocabularies.
An unusual very low-mass high-amplitude pre-main sequence periodic variable
Maria V. Rodriguez-Ledesma,Reinhard Mundt,Mansur Ibrahimov,Sergio Messina,Padmakar Parihar,Frederic Hessman,Catarina Alves de Oliveira,William Herbst
Physics , 2012, DOI: 10.1051/0004-6361/201118649
Abstract: We have investigated the nature of the variability of CHS7797, an unusual periodic variable in the Orion Nebula Cluster. An extensive I-band photometric data set of CHS7797 was compiled between 2004-2010 using various telescopes. Further optical data have been collected in R and z' bands. In addition, simultaneous observations of the ONC region including CHS7797 were performed in the I, J, Ks and IRAC [3.6] and [4.5] bands over a time interval of about 40d. CHS7797 shows an unusual large-amplitude variation of about 1.7 mag in the R, I, and z' bands with a period 17.786. The amplitude of the brightness modulation decreases only slightly at longer wavelengths. The star is faint during 2/3 of the period and the shape of the phased light-curves for seven different observing seasons shows minor changes and small-amplitude variations. Interestingly, there are no significant colour-flux correlations for wavelengths smaller than 2microns, while the object becomes redder when fainter at longer wavelengths. CHS7797 has a spectral type of M6 and an estimated mass between 0.04-0.1Msun. The analysis of the data suggests that the periodic variability of CHS7797 is most probably caused by an orbital motion. Variability as a result of rotational brightness modulation by spots is excluded by the lack of any color-brightness correlation in the optical. The latter indicates that CHS7797 is most probably occulted by circumstellar matter in which grains have grown from typical 0.1 microns to 1-2 micron sizes. We discuss two possible scenarios in which CHS7797 is periodically eclipsed by structures in a disc, namely that CHS7797 is a single object with a circumstellar disc, or that CHS7797 is a binary system, similar to KH15D, in which an inclined circumbinary disc is responsible of the variability. Possible reasons for the typical 0.3mag variations in I-band at a given phase are discussed.
Detecting circumstellar disks around gravitational microlenses
M. Hundertmark,F. V. Hessman,S. Dreizler
Physics , 2009, DOI: 10.1051/0004-6361/200811458
Abstract: We investigate the chance of detecting proto-planetary or debris disks in stars that induce microlensing events (lenses). The modification of the light curves shapes due to occultation and extinction by the disks as well as the additional gravitational deflection caused by the additional mass is considered. The magnification of gravitational microlensing events is calculated using the ray shooting method. The occultation is taken into account by neglecting or weighting the images on the lens plane according to a transmission map of the corresponding disk for a point source point lens (PSPL) model. The estimated frequency of events is obtained by taking the possible inclinations and optical depths of the disk into account. We conclude that gravitational microlensing can be used, in principle, as a tool for detecting debris disks beyond 1 kpc, but estimate that each year of the order of 1 debris disk is expected for lens stars of F, G, or K spectral type and of the order of 10 debris disks might have shown signatures in existing datasets.
Second generation planet formation in NN Serpentis?
M. V?lschow,R. Banerjee,F. V. Hessman
Physics , 2013, DOI: 10.1051/0004-6361/201322111
Abstract: In this paper, we study the general impact of stellar mass-ejection events in planetary orbits in post-common envelope binaries with circumbinary planets like those around NN Serpentis. We discuss a set of simple equations that determine upper and lower limits for orbital expansion and investigate the effect of initial eccentricity. We deduce the range of possible semi-major axes and initial eccentricity values of the planets prior to the common-envelope event. In addition to spherically-symmetric mass-ejection events, we consider planetary dynamics under the influence of an expanding disk. In order to have survived, we suggest that the present planets in NN Ser must have had semi-major axes $\,{}^>_{\sim}\, 10$ AU and high eccentricity values which is in conflict with current observations. Consequently, we argue that these planets were not formed together with their hosting stellar system, but rather originated from the fraction of matter of the envelope that remained bound to the binary. According to the cooling age of the white dwarf primary of $10^6$ yr, the planets around NN Ser might be the youngest known so far and open up a wide range of further study of second generation planet formation.
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