Journals
2009 EN
Steven H. Pravdo · Stuart B. Shaklan
We report here the discovery of the first planet around an ultracool dwarfstar. It is also the first extrasolar giant planet (EGP) astrometricallydiscovered around a main-sequence star. The statistical significance of thedetection is shown in two ways. First, there is a 2 x 10^-8 probability thatthe astrometric motion fits a parallax-and-proper-motion-only model. Second,periodogram analysis shows a false alarm probability of 3 x 10^-5 that thediscovered period is randomly generated. The planetary mass is M2 = 6.4(+2.6,-3.1) Jupiter-masses (MJ), and the orbital period is P = 0.744(+0.013,-0.008) yr in the most likely model. In less likely models, companionmasses that are higher than the 13 MJ planetary mass limit are ruled out bypast radial velocity measurements unless the system radial velocity is morethan twice the current upper limits and the near-periastron orbital phase wasnever observed. This new planetary system is remarkable, in part, because itsstar, VB 10, is near the lower mass limit for a star. Our astrometricobservations provide a dynamical mass measurement and will in time allow us toconfront the theoretical models of formation and evolution of such systems andtheir members. We thus add to the diversity of planetary systems and to thesmall number of known M-dwarf planets. Planets such as VB 10b could be the mostnumerous type of planets because M stars comprise >70% of all stars. To datethey have remained hidden since the dominant radial-velocity (RV)planet-discovery technique is relatively insensitive to these dim, red systems.
Journals
2009 EN
Joleen K. Carlberg · Steven R. Majewski · Phil Arras
Rapid rotation in field red giant stars is a relatively rare but well-studiedphenomenon; here we investigate the potential role of planet accretion inspinning up these stars. Using Zahn's theory of tidal friction and stellarevolution models, we compute the decay of a planet's orbit into its evolvinghost star and the resulting transfer of angular momentum into the stellarconvective envelope. This experiment assesses the frequency of planet ingestionand rapid rotation on the red giant branch (RGB) for a sample of 99 knownexoplanet host stars. We find that the known exoplanets are indeed capable ofcreating rapid rotators; however, the expected fraction due to planet ingestionis only ~10% of the total seen in surveys of present-day red giants. Of theplanets ingested, we find that those with smaller initial semimajor axes aremore likely to create rapid rotators because these planets are accreted whenthe stellar moment of inertia is smallest. We also find that many planets maybe ingested prior to the RGB phase, contrary to the expectation that accretionwould generally occur when the stellar radii expand significantly as giants.Finally, our models suggest that the rapid rotation signal from ingestedplanets is most likely to be seen on the lower RGB, which is also wherealternative mechanisms for spin-up, e.g., angular momentum dredged up from thestellar core, do not operate. Thus, rapid rotators on the lower RGB are thebest candidates to search for definitive evidence of systems that haveexperienced planet accretion.
Journals
2009 EN
ETH Zurich Simon Bruderer · ETH Zurich Arnold O. Benz · Denison University Steven D. Doty
+2 more
Observations of the high-mass star forming region AFGL 2591 reveal a largeabundance of CO+, a molecule known to be enhanced by far UV (FUV) and X-rayirradiation. In chemical models assuming a spherically symmetric envelope, thevolume of gas irradiated by protostellar FUV radiation is very small due to thehigh extinction by dust. The abundance of CO+ is thus underpredicted by ordersof magnitude. In a more realistic model, FUV photons can escape through anoutflow region and irradiate gas at the border to the envelope. Thus, weintroduce the first 2D axi-symmetric chemical model of the envelope of ahigh-mass star forming region to explain the CO+ observations as a prototypicalFUV tracer. The model assumes an axi-symmetric power-law density structure witha cavity due to the outflow. The local FUV flux is calculated by a Monte Carloradiative transfer code taking scattering on dust into account. A grid ofprecalculated chemical abundances, introduced in the first part of this seriesof papers, is used to quickly interpolate chemical abundances. This approachallows to calculate the temperature structure of the FUV heated outflow wallsself-consistently with the chemistry. Synthetic maps of the line flux are calculated using a raytracer code.Single-dish and interferometric observations are simulated and the modelresults are compared to published and new JCMT and SMA observations. Thetwo-dimensional model of AFGL 2591 is able to reproduce the JCMT single-dishobservations and also explains the non-detection by the SMA. We conclude thatthe observed CO+ line flux and its narrow width can be interpreted by emissionfrom the warm and dense outflow walls irradiated by protostellar FUV radiation.
Journals
2009 EN
UCLA Steven Furlanetto
After quasars ionize intergalactic HeII at z~3, a large radiation fieldbuilds up above the HeII ionization edge. Unlike the background responsible forHI ionizations, this field should be highly variable, thanks to the scarcity ofbright quasars and the relatively short attenuation lengths (~50 Mpc) of thesehigh-energy photons. Recent observations of the HeII and HI Lyman-alpha forestsshow that this background does indeed vary strongly, with substantialfluctuations on scales as small as ~2 Mpc. Here we show that such spatialfluctuation scales are naturally expected in any model in which the sources areas rare as bright quasars, so long as the attenuation length is relativelysmall. The correlation length itself is comparable to the attenuation length(~10 Mpc) for the most plausible physical scenarios, but we findorder-of-magnitude fluctuations on all scales smaller than ~6 Mpc. Moreover,aliasing along the one-dimensional skewers probed by the HeII and HILyman-alpha forests exaggerates these variations, so that order-of-magnitudefluctuations should be observed on all scales smaller than ~20 Mpc. Complexradiative transfer is therefore not required to explain the observedfluctuations, at least at the level of current data.
Journals
2009 EN
Harvard-Smithsonian CfA Steven R. Cranmer
Classical Be stars are an enigmatic subclass of rapidly rotating hot starscharacterized by dense equatorial disks of gas that have been inferred to orbitwith Keplerian velocities. Although these disks seem to be ejected from thestar and not accreted, there is substantial observational evidence to show thatthe stars rotate more slowly than required for centrifugally driven mass loss.This paper develops an idea (proposed originally by Hiroyasu Ando andcolleagues) that nonradial stellar pulsations inject enough angular momentuminto the upper atmosphere to spin up a Keplerian disk. The pulsationsthemselves are evanescent in the stellar photosphere, but they may be unstableto the generation of resonant oscillations at the acoustic cutoff frequency. Adetailed theory of the conversion from pulsations to resonant waves does notyet exist for realistic hot-star atmospheres, so the current models depend on aparameterized approximation for the efficiency of wave excitation. Onceresonant waves have been formed, however, they grow in amplitude withincreasing height, steepen into shocks, and exert radial and azimuthal Reynoldsstresses on the mean fluid. Using reasonable assumptions for the stellarparameters, these processes were found to naturally create the inner boundaryconditions required for dense Keplerian disks, even when the underlying B-starphotosphere is rotating as slowly as 60% of its critical rotation speed.Because there is evidence for long-term changes in Be-star pulsationalproperties, this model may also account for the long-term variability of Bestars, including transitions between normal, Be, and shell phases.
Journals
2009 EN
UCLA Steven Furlanetto · UCSB S. Peng Oh
We use an analytic model to study how inhomogeneous hydrogen reionizationaffects the temperature distribution of the intergalactic medium (IGM). Duringthis process, the residual energy of each ionizing photon is deposited in theIGM as heat, increasing its temperature to 20,000-30,000 K; subsequentexpansion of the Universe then cools the gas. Because reionization most likelyproceeds from high to low densities, underdense voids are ionized last, haveless time to cool, and are (on average) warmer than mean-density gasimmediately after reionization is complete (an "inverted" equation of state). From this initial configuration, the low-density gas cools quickly andeventually returns to a more normal equation of state. The rapidly evolvingtemperature introduces systematic uncertainties in measurements of the ionizingbackground at z~6. For example, late reionization implies rapid cooling, sothat the ionizing background would have to evolve even more rapidly at z ~5-6than typically claimed. This degeneracy is difficult to disentangle, becausethe Lyman-alpha forest probes only a narrow range in densities (over which thegas is nearly isothermal). However, higher Lyman-series transitions probe widerdensity ranges, sampling different effective temperatures, and offer a new wayto measure the IGM equation of state that should work where nearly saturatedabsorption precludes other methods. This will help to separate evolution intemperature from that in the ionizing background. While more detailed studywith hydrodynamic simulations is needed, we show that such measurements couldpotentially distinguish early and late reionization using only a handful oflines of sight.
Journals
2009 EN
Dana I. Casetti-Dinescu · Terrence M. Girard · Steven R. Majewski
+5 more
We present absolute proper motions in Kapteyn Selected Area (SA) 103. Thisfield is located 7 degrees west of the center of the Virgo Stellar Stream (VSS,Duffau et al. 2006), and has a well-defined main sequence representing thestream. In SA 103 we identify one RR Lyrae star as a member of the VSSaccording to its metallicity, radial velocity and distance. VSS candidateturnoff stars and subgiant stars have proper motions consistent with that ofthe RR Lyrae star. The 3D velocity data imply an orbit with a pericenter of 11kpc and an apocenter of ~90 kpc. Thus, the VSS comprises tidal debris foundnear the pericenter of a highly destructive orbit. Examining the six globularclusters at distances larger than 50 kpc from the Galactic center, and theproposed orbit of the VSS, we find one tentative association, NGC 2419. Wespeculate that NGC 2419 is possibly the nucleus of a disrupted system of whichthe VSS is a part.
Journals
2009 EN
Jeffrey L. Carlin · Carl J. Grillmair · Ricardo R. Munoz
+2 more
We report the results of a spectroscopic study of the Bootes III (BooIII)stellar overdensity carried out with the Hectospec multifiber spectrograph onthe MMT telescope. Radial velocities have been measured for 193 BooIIIcandidate stars selected to have magnitudes and colors consistent with itsupper main sequence and lower red giant branch, as well as a number ofhorizontal branch candidates. From 20 identified candidate BooIII members, wemeasure a systemic velocity of V_sun=197.5+-3.8 km/s and a velocity dispersionof sigma_o=14.0+-3.2 km/s. We use the somewhat large velocity dispersion andthe implied highly radial orbit, along with morphological evidence fromGrillmair (2009) and stellar abundances, to argue that BooIII is likely thefirst known object observed in a transitional state between being a bound dwarfgalaxy and a completely unbound tidal stream.
Journals
2009 EN
Carnegie Observatories Joshua D. Simon · Weizmann
Institute Avishay Gal-Yam · Caltech Orly Gnat
+18 more
Recent observations have revealed that some Type Ia supernovae exhibitnarrow, time-variable Na I D absorption features. The origin of the absorbingmaterial is controversial, but it may suggest the presence of circumstellar gasin the progenitor system prior to the explosion, with significant implicationsfor the nature of the supernova progenitors. We present the third detection ofsuch variable absorption, based on six epochs of high-resolution spectroscopyof the Type Ia supernova SN 2007le from Keck and the HET. The data span ~3months, from 5 days before maximum light to 90 days after maximum. We find thatone component of the Na D absorption lines strengthened significantly withtime, indicating a total column density increase of ~2.5 x 10^12 cm^-2. Thechanges are most prominent after maximum light rather than at earlier timeswhen the UV flux from the SN peaks. As with SN 2006X, we detect no change inthe Ca II H&K lines over the same time period, rendering line-of-sight effectsimprobable and suggesting a circumstellar origin for the absorbing material.Unlike the previous two SNe exhibiting variable absorption, SN 2007le is nothighly reddened (E_B-V = 0.27 mag), also pointing toward circumstellar ratherthan interstellar absorption. Photoionization models show that the data areconsistent with a dense (10^7 cm^-3) cloud or clouds of gas located ~0.1 pcfrom the explosion. These results broadly support the single-degeneratescenario previously proposed to explain the variable absorption, with mass lossfrom a nondegenerate companion star responsible for providing the circumstellargas. We also present tentative evidence for narrow Halpha emission associatedwith the SN, which will require followup observations at late times to confirm.[abridged]
Journals
2009 EN
University of Waterloo Steven P. Allanson · University of Waterloo Michael J. Hudson · Durham University Russell J. Smith
+1 more
This paper addresses the challenge of understanding the typical starformation histories of red sequence galaxies, using linestrength indices andmass-to-light ratios as complementary constraints on their stellar agedistribution. We construct simple parametric models of the star formationhistory that bracket a range of scenarios, and fit these models to thelinestrength indices of low-redshift cluster red-sequence galaxies. For giantgalaxies, we confirm the downsizing trend. We find, however, that this trendflattens or reverses at sigma < 70 km/s. We then compare predicted stellarmass-to-light ratios with dynamical mass-to-light ratios derived from theFundamental Plane (FP), or by the SAURON group. For galaxies with sigma ~ 70km/s, models with a "frosting" of young stars and models with exponential starformation histories have stellar mass-to-light ratios that are larger thanobserved dynamical mass-to-light ratios by factors of 1.7 and 1.4,respectively, and so are rejected. The SSP model is consistent with the FP, andrequires a modest amount of dark matter (20-30%) to account for the differencebetween stellar and dynamical mass-to-light ratios. A model in which starformation was "quenched" at intermediate ages is also consistent with theobservations. We find that the contribution of stellar populations to the"tilt" of the FP is highly dependent on the assumed star-formation history: forthe SSP model, the tilt of the FP is driven primarily by stellar-populationeffects. For a quenched model, two-thirds of the tilt is due to stellarpopulations and only one third is due to dark matter or non-homology.