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The Harvard–Smithsonian Center for Astrophysics is one of the largest and most diverse astrophysical institutions in the world, where scientists carry out a broad program of research in astronomy, astrophysics, earth and space science, and science education. The center's mission is to advance knowledge and understanding of the universe through research and education in astronomy and astrophysics.The center was founded in 1973 as a joint venture between the Smithsonian Institution and Harvard University. It consists of the Harvard College Observatory and the Smithsonian Astrophysical Observatory. The center's main facility is located between Concord Avenue and Garden Street, with its mailing address and main entrance at 60 Garden Street, Cambridge, Massachusetts. Beyond this location there are also additional satellite facilities elsewhere around the globe. The current director of the CfA, Charles R. Alcock, was named in 2004. The director from 1982 to 2004 was Irwin I. Shapiro. Wikipedia.

Kipping D.M.,Harvard - Smithsonian Center for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

Stellar limb darkening affects a wide range of astronomical measurements and is frequently modelled with a parametric model using polynomials in the cosine of the angle between the line of sight and the emergent intensity. Two-parameter laws are particularly popular for cases where one wishes to fit freely for the limb darkening coefficients (i.e. an uninformative prior) due to the compact prior volume and the fact that more complex models rarely obtain unique solutions with the present data. In such cases, we show that the two limb darkening coefficients are constrained by three physical boundary conditions, describing a triangular region in the two-dimensional parameter space.We show that uniformly distributed samples may be drawn from this region with optimal efficiency by a technique developed by computer graphical programming: triangular sampling. Alternatively, one can make draws using a uniform, bivariate Dirichlet distribution.We provide simple expressions for these parametrizations for both techniques applied to the case of quadratic, square-root and logarithmic limb darkening laws. For example, in the case of the popular quadratic law, we advocate fitting for q1 = (u1 + u2)2 and q2 = 0.5u1(u1 + u2)-1 with uniform priors in the interval [0, 1] to implement triangular sampling easily. Employing these parametrizations allows one to derive model parameters which fully account for our ignorance about the intensity profile, yet never explore unphysical solutions, yielding robust and realistic uncertainty estimates. Furthermore, in the case of triangular sampling with the quadratic law, our parametrization leads to significantly reduced mutual correlations and provides an alternative geometric explanation as to why naively fitting the quadratic limb darkening coefficients precipitates strong correlations in the first place. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Youdin A.N.,Harvard - Smithsonian Center for Astrophysics
Astrophysical Journal | Year: 2011

We develop a general method to fit the underlying planetary distribution function (PLDF) to exoplanet survey data. This maximum likelihood method accommodates more than one planet per star and any number of planet or target star properties. We apply the method to announced Kepler planet candidates that transit solar-type stars. The Kepler team's estimates of the detection efficiency are used and are shown to agree with theoretical predictions for an ideal transit survey. The PLDF is fit to a joint power law in planet radius, down to 0.5 R ⊕, and orbital period, up to 50days. The estimated number of planets per star in this sample is ∼0.7-1.4, where the range covers systematic uncertainties in the detection efficiency. To analyze trends in the PLDF we consider four planet samples, divided between shorter and longer periods at 7days and between large and small radii at 3 R ⊕. The size distribution changes appreciably between these four samples, revealing a relative deficit of ∼3 R⊕ planets at the shortest periods. This deficit is suggestive of preferential evaporation and sublimation of Neptune- and Saturn-like planets. If the trend and explanation hold, it would be spectacular observational support of the core accretion and migration hypotheses, and would allow refinement of these theories. © 2011. The American Astronomical Society. All rights reserved.. Source

Kipping D.M.,Harvard - Smithsonian Center for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2014

Eclipsing systems, such as transiting exoplanets, allow one to measure the mean stellar density of the host star under various idealized assumptions. Asterodensity profiling (AP) compares this density to an independently determined value in order to check the validity of the assumptions and ultimately derive useful parameters. Several physical effects can cause said assumptions to become invalid, with the most well-known example being the so-called photoeccentric effect. In thiswork,we provide analytic expressions for five other effectswhich induce AP deviations: the photoblend, -spot, -timing, -duration and -mass effects.We find that these effects can easily reproduce large AP deviations and so we caution that extracting the eccentricity distribution is only viable with careful consideration of the prior distributions for these other effects. We also re-investigate the photoeccentric effect and derive a single-domain minimum eccentricity expression and the parameter range for which analytic formulae are valid. The latter result shows that the assumptions underlying the analytic model for the photoeccentric effect break down for close-in, highly eccentric planets, meaning that extreme care must be taken in this regime. Finally, we demonstrate that contaminated light fraction can be solved for, indicating that AP could be a potent tool for planet validation. © 2014 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Yuan F.,Chinese Academy of Sciences | Narayan R.,Harvard - Smithsonian Center for Astrophysics
Annual Review of Astronomy and Astrophysics | Year: 2014

Black hole accretion flows can be divided into two broad classes: cold and hot. Whereas cold accretion flows consist of cool optically thick gas and are found at relatively high mass accretion rates, hot accretion flows, the topic of this review, are virially hot and optically thin, and occur at lower mass accretion rates. They are described by accretion solutions such as the advection-dominated accretion flow and luminous hot accretion flow. Because of energy advection, the radiative efficiency of these flows is in general lower than that of a standard thin accretion disk. Moreover, the efficiency decreases with decreasing mass accretion rate. Observations show that hot accretion flows are associated with jets. In addition, theoretical arguments suggest that hot flows should produce strong winds. Hot accretion flows are believed to be present in low-luminosity active galactic nuclei and in black hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A, the ultralow-luminosity supermassive black hole at our Galactic center. The jet, wind, and radiation from a supermassive black hole with a hot accretion flow can interact with the external interstellar medium and modify the evolution of the host galaxy. Copyright © 2014 by Annual Reviews. Source

Berger E.,Harvard - Smithsonian Center for Astrophysics
Annual Review of Astronomy and Astrophysics | Year: 2014

Gamma-ray bursts (GRBs) display a bimodal duration distribution with a separation between the short- and long-duration bursts at about 2 s. The progenitors of long GRBs have been identified as massive stars based on their association with Type Ic core-collapse supernovae (SNe), their exclusive location in star-forming galaxies, and their strong correlation with bright UV regions within their host galaxies. Short GRBs have long been suspected on theoretical grounds to arise from compact object binary mergers (neutron star-neutron star or neutron star-black hole). The discovery of short GRB afterglows in 2005 provided the first insight into their energy scale and environments, as well as established a cosmological origin, a mix of host-galaxy types, and an absence of associated SNe. In this review, I summarize nearly a decade of short GRB afterglow and host-galaxy observations and use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments. The preponderance of the evidence points to compact object binary progenitors, although some open questions remain. On the basis of this association, observations of short GRBs and their afterglows can shed light on the on- and off-axis electromagnetic counterparts of gravitational wave sources from the Advanced LIGO/Virgo experiments. Copyright © 2014 by Annual Reviews. Source

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