Angular Momentum Evolution of Stars in the Orion Nebula Cluster

(Abridged) We present theoretical models of stellar angular momentum evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades. We demonstrate that observations of the Pleiades and Hyades place tight constraints on the angular momentum loss rate from stellar winds. The observed periods, masses and ages of ONC stars in the range 0.2--0.5 M$_\odot$, and the loss properties inferred from the Pleiades and Hyades stars, are then used to test the initial conditions for stellar evolution models. We use these models to estimate the distribution of rotational velocities for the ONC stars at the age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades distributions of rotation rates are not consistent if only stellar winds are included. In order to reconcile the observed loss of angu lar momentum between these two clusters, an extrinsic loss mechanism such as protostar-accretion disk interaction is required. Our model, which evolves the ONC stars with a mass dependent saturation threshold normalized such that $\omega_{crit} = 5.4 \omega_\odot$ at 0.5 \m, and which includes a distribution of disk lifetimes that is uniform over the range 0--6 Myr, is consistent with the Pleiades. This model for disk-locking lifetimes is also consistent with inferred disk lifetimes from the percentage of stars with infrared excesses observed in young clusters. Different models, using a variety of initial period distributions and different maximum disk lifetimes, are also compared to the Pleiades. For disk-locking models that use a uniform distribution of disk lifetimes over the range 0 to $\tau_{max}$, the acceptable range of the maximum lifetime is $3.5 < \tau_{max} < 8.5$ Myr.Comment: 21 pages, 7 figures, submitted to Ap

Experimental and numerical analysis of deformation patterns in notched heterogeneous welds

Standardized weld flaw assessment techniques assume the weld region to be homogeneous which is a strong idealisation of reality. Characterising the effects of heterogeneous properties of welds through the analysis of deformation patterns and slip lines is the major concern of this research. It is the goal to investigate which effects these variations in properties within the weld material have on the propagation of cracks within the weld material. Performed experiments are SENT tests on strongly heterogeneous welded connections. The same material is also simulated with a weld heterogenisation model in ABAQUS®. Results from both experiments and simulations are discussed and compared. It is shown that slip lines tend to avoid zones of high hardness in a way that a path of least resistance is found. Related to this, it is seen that the slip line angles deviate from the theoretical 45° for homogeneous material. Obtained results validate the numerical model used

Verkeersveiligheid op vrije tram- en busbanen

In augustus 2007 werd het bestek nr. ABMV/07/VV/VTBB Studieopdracht “Verkeersveiligheid op vrije tram- en busbanen” gepubliceerd. Het doel van deze opdracht was de verkeersveiligheid van vrije tram- en busbanen in beeld te brengen en te analyseren. Daarbij diende vertrokken te worden van een gedetailleerde, multidisciplinaire analyse van de ongevallen, geregistreerd op de Vlaamse beddingen voor het openbaar vervoer, en van de identificatie van mogelijke ongevals-oorzaken. Bovendien zouden uit het onderzoek een aantal algemene richtlijnen,moeten geformuleerd worden voor de (her)aanleg van tram- en busbanen. Meer specifiek wil men met dit onderzoek een antwoord krijgen op volgende vier vragen: 1. Hoe is het gesteld met de verkeersveiligheid op vrije tram- en busbanen? 2. Komt de aanleg van vrije tram- en busbanen in conflict met de veiligheid van de zwakke weggebruiker? Is die onveiligheid groter dan op anders ingerichte wegen met vergelijkbare functie? 3. Welke factoren liggen ten grondslag aan de verkeersongevallen op vrije tram- en busbanen? 4. Welke maatregelen kunnen genomen worden om de verkeers-veiligheid op vrije tram- en busbanen te verbeteren? Welke richtlijnen kunnen geformuleerd worden voor de aanleg van vrije tram- en busbanen vanuit verkeersveiligheidsoogpunt? De vier verschillende fases van het onderzoek werden in het bestek gedetailleerd beschreven: In een eerste fase dient een globale analyse van de verkeersveiligheid op weggedeelten met vrije tram- en busbanen te gebeuren, en dit aan de hand van een gedetailleerde inventaris van de geregistreerde ongevallen. In een tweede fase dient de vastgestelde onveiligheid van verschillende tram- en busbanen te worden vergeleken met de verkeersveiligheid op anders ingerichte, doch qua functie vergelijkbare weggedeelten. Daartoe dient te worden uitgegaan van de frequentie van ongevallen en van de vergelijking van de oorzaken van de ongevallen op beide “types” weginrichting. In een derde fase volgt een probleemanalyse. Deze probleemanalyse baseert zich in eerste instantie op de ongevalgegevens verzameld tijdens de eerste fase om mogelijke conflictsituaties in beeld te brengen. Daarnaast worden specifieke knelpunten geïdentificeerd, die een veilige interactie tussen de voertuigen van het openbaar vervoer (bussen en trams) en de andere weggebruikers, voornamelijk de zwakke weggebruikers (zoals voetgangers en fietsers), bemoeilijken. Deze knelpunten kunnen gesitueerd zijn op de tram- en/of busbaan zelf, in de onmiddellijke omgeving ervan of zelfs ruimer, dit is in de algehele inrichting van de straat en/of de ordening van de verkeers-afwikkeling in die zone. Meer specifiek dient dus nagegaan te worden welke factoren ten grondslag liggen aan verkeersongevallen op vrije tram- en busbanen. Deze factoren kunnen zowel gedrags- als omgevingsfactoren omvatten. Gedragsfactoren kunnen betrekking hebben op het gedrag van de tram- en busbestuurders alsook op het gedrag van de andere weggebruikers op en rond de vrije tram- en/of busbaan. Fysieke kenmerken van vrije tram- en busbanen die een invloed kunnen hebben op de verkeersveiligheid dienen eveneens nader bekeken te worden. Bijzondere aandacht dient ook uit te gaan naar de problematiek van de oversteekplaatsen op de vrije tram- en busbanen. In een vierde fase dienen, naast formulering van beleidsaanbevelingen voortvloeiend uit de probleemanalyse, algemene richtlijnen voor de aanleg van tram- en busbanen geformuleerd te worden. Hierbij kan een onderscheid gemaakt worden tussen de aanleg van nieuwe tram- en busbanen en de heraanleg van bestaande tram- en busbanen. De richtlijnen moeten betrekking hebben op beide situaties en oog hebben voor het feit dat de inrichtingen steeds het gevolg zullen zijn van maatwerk, steeds in te passen in een andere ruimtelijke (en verkeerskundige) omgeving

A Search for Photometric Rotation Periods in Low-Mass Stars and Brown Dwarfs in the Pleiades

We have photometrically monitored (Cousins Ic) eight low mass stars and brown dwarfs which are probable members of the Pleiades. We derived rotation periods for two of the stars - HHJ409 and CFHT-PL8 - to be 0.258 d and 0.401 d, respectively. The masses of these stars are near 0.4 and 0.08 Msun, respectively; the latter is the second such object near the hydrogen-burning boundary for which a rotation period has been measured. We also observed HHJ409 in V; the relative amplitude in the two bands shows that the spots in that star are about 200 K cooler than the stellar effective temperature of 3560 K and have a filling factor on the order of 13%. With one possible exception, the remaining stars in the sample do not show photometric variations larger than the mean error of measurement. We also examined the M9.5V disk star 2MASSJ0149, which had previously exhibited a strong flare event, but did not detect any photometric variation.Comment: 13 pages, four figures. Accepted for publication in A

Angular Momentum Transport In Solar-Type Stars: Testing the Timescale For Core-Envelope Coupling

We critically examine the constraints on internal angular momentum transport which can be inferred from the spin down of open cluster stars. The rotation distribution inferred from rotation velocities and periods are consistent for larger and more recent samples, but smaller samples of rotation periods appear biased relative to vsini studies. We therefore focus on whether the rotation period distributions observed in star forming regions can be evolved into the observed ones in the Pleiades, NGC2516, M34, M35, M37, and M50 with plausible assumptions about star-disk coupling and angular momentum loss from magnetized solar-like winds. Solid body models are consistent with the data for low mass fully convective stars but highly inconsistent for higher mass stars where the surface convection zone can decouple for angular momentum purposes from the radiative interior. The Tayler-Spruit magnetic angular momentum transport mechanism, commonly employed in models of high mass stars, predicts solid-body rotation on extremely short timescales and is therefore unlikely to operate in solar-type pre-MS and MS stars at the predicted rate. Models with core-envelope decoupling can explain the spin down of 1.0 and 0.8 solar mass slow rotators with characteristic coupling timescales of 55+-25 Myr and 175+-25 Myr respectively. The upper envelope of the rotation distribution is more strongly coupled than the lower envelope of the rotation distribution, in accord with theoretical predictions that the angular momentum transport timescale should be shorter for more rapidly rotating stars. Constraints imposed by the solar rotation curve are also discussed (Abridged)Comment: 42 pages, 16 figures, submitted to Ap

Rotational velocities of low-mass stars in the Pleiades and Hyades

We have obtained high-resolution spectra of 89 M dwarf members of the Pleiades and Hyades and have derived radial velocities, H-alpha equivalent widths, and spectroscopic rotational velocities for these stars. Typical masses of the newly-observed Pleiades and Hyades stars are ~ 0.4 M_{\sun} and ~ 0.2 M_{\sun}, respectively. We combine our new observations with previously published data to explore the rotational evolution of young stars with M < 0.4 M_\sun. The average rotation rate in the Hyades (age 600 Myr) is about 0.4 that of the Pleiades (110 Myr), and the mean equivalent widths of H-alpha are also lower. As found in previous studies, the correlation between rotation and chromospheric activity is identical in both clusters, implying that the lower activity in the Hyades is a result of the lower rotation rates. We show that a simple scaling of the Pleiades rotational distribution for M \leq 0.4 M_{\sun}, corrected for the effects of structural evolution, matches that of the Hyades if the average angular momentum loss from the Pleiades to the Hyades age is factor of \approx 6. This suggests that the distribution of initial angular momenta and disk-locking lifetimes for the lowest mass stars was similar in both clusters. We argue that this result provides further evidence for a saturation of the angular momentum loss rate at high rotational velocities.Comment: 22 pages, 11 figures, accepted for publication in The Astronomical Journal, tentatively scheduled for March 200

How Good a Clock is Rotation? The Stellar Rotation-Mass-Age Relationship for Old Field Stars

The rotation-mass-age relationship offers a promising avenue for measuring the ages of field stars, assuming the attendant uncertainties to this technique can be well characterized. We model stellar angular momentum evolution starting with a rotation distribution from open cluster M37. Our predicted rotation-mass-age relationship shows significant zero-point offsets compared to an alternative angular momentum loss law and published gyrochronology relations. Systematic errors at the 30 percent level are permitted by current data, highlighting the need for empirical guidance. We identify two fundamental sources of uncertainty that limit the precision of rotation-based ages and quantify their impact. Stars are born with a range of rotation rates, which leads to an age range at fixed rotation period. We find that the inherent ambiguity from the initial conditions is important for all young stars, and remains large for old stars below 0.6 solar masses. Latitudinal surface differential rotation also introduces a minimum uncertainty into rotation period measurements and, by extension, rotation-based ages. Both models and the data from binary star systems 61 Cyg and alpha Cen demonstrate that latitudinal differential rotation is the limiting factor for rotation-based age precision among old field stars, inducing uncertainties at the ~2 Gyr level. We also examine the relationship between variability amplitude, rotation period, and age. Existing ground-based surveys can detect field populations with ages as old as 1-2 Gyr, while space missions can detect stars as old as the Galactic disk. In comparison with other techniques for measuring the ages of lower main sequence stars, including geometric parallax and asteroseismology, rotation-based ages have the potential to be the most precise chronometer for 0.6-1.0 solar mass stars.Comment: For a brief video explaining the key results of this paper, see http://www.youtube.com/user/OSUAstronom

Rotation and activity in the solar-metallicity open cluster NGC2516

We report new measures of radial velocities and rotation rates (v sin i) for 51 F and early-G stars in the open cluster NGC2516, and combine these with previously published data. From high signal-to-noise spectra of two stars, we show that NGC2516 has a relative iron abundance with respect to the Pleiades of delta([Fe/H])= +0.04 +/- 0.07 at the canonical reddening of E(B - V) = 0.12, in contrast to previous photometric studies that placed the cluster 0.2 to 0.4 dex below solar. We construct a color-magnitude diagram based on radial velocity members, and explore the sensitivity of photometric determinations of the metallicity and distance to assumed values of the reddening. For a metal abundance near solar, the Hipparcos distance to NGC2516 is probably underestimated. Finally, we show that the distribution of rotation rates and X-ray emission does not differ greatly from that of the Pleiades, when allowance is made for the somewhat older age of NGC2516.Comment: Accepted for publication in the Astrophysical Journal. 35 pages including 7 figure

Validation practices for satellite based earth observation data across communities

Assessing the inherent uncertainties in satellite data products is a challenging task. Different technical approaches have been developed in the Earth Observation (EO) communities to address the validation problem which results in a large variety of methods as well as terminology. This paper reviews state-of-the-art methods of satellite validation and documents their similarities and differences. First the overall validation objectives and terminologies are specified, followed by a generic mathematical formulation of the validation problem. Metrics currently used as well as more advanced EO validation approaches are introduced thereafter. An outlook on the applicability and requirements of current EO validation approaches and targets is given