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Roe H.G.,Lowell Observatory
Annual Review of Earth and Planetary Sciences | Year: 2012

Conditions in Titan's troposphere are near the triple point of methane, the second most abundant component of its atmosphere. Our understanding of Titan's lower atmosphere has shifted considerably in the past decade. Ground-based observations, Hubble Space Telescope images, and data returned from the Cassini and Huygens spacecraft show that Titan's troposphere hosts a methane-based meteorology in direct analogy to the water-based meteorology of Earth. What once was thought to be a quiescent place, lacking in clouds or localized weather and changing only subtly on long seasonal timescales, is now understood to be a dynamic system with significant weather events regularly occurring against the backdrop of dramatic seasonal changes. Although the observational record of Titan's weather covers only a third of its 30-year seasonal cycle, Titan's atmospheric processes appear to be more closely analogous to those of Earth than to those of any other object in our solar system. © 2012 by Annual Reviews. All rights reserved.


Massey P.,Lowell Observatory
New Astronomy Reviews | Year: 2013

The star-forming galaxies of the Local Group act as our laboratories for testing massive star evolutionary models. In this review, I briefly summarize what we believe we know about massive star evolution, and the connection between OB stars, Luminous Blue Variables, yellow supergiants, red supergiants, and Wolf-Rayet stars. The difficulties and recent successes in identifying these various types of massive stars in the neighboring galaxies of the Local Group will be discussed. © 2013 Elsevier B.V.


Shkolnik E.L.,Lowell Observatory
Astrophysical Journal | Year: 2013

Using the far-UV (FUV) and near-UV (NUV) photometry from the NASA Galaxy Evolution Explorer (GALEX), we searched for evidence of increased stellar activity due to tidal and/or magnetic star-planet interactions (SPI) in the 272 known FGK planetary hosts observed by GALEX. With the increased sensitivity of GALEX, we are able probe systems with lower activity levels and at larger distances than what has been done to date with X-ray satellites. We compared samples of stars with close-in planets (a < 0.1 AU) to those with far-out planets (a > 0.5 AU) and looked for correlations of excess activity with other system parameters. This statistical investigation found no clear correlations with a, Mp , or Mp /a, in contrast to some X-ray and Ca II studies. However, there is tentative evidence (at a level of 1.8σ) that stars with radial-velocity-(RV)-detected close-in planets are more FUV-active than stars with far-out planets, in agreement with several published X-ray and Ca II results. The case is strengthened to a level of significance to 2.3σ when transit-detected close-in planets are included. This is most likely because the RV-selected sample of stars is significantly less active than the field population of comparable stars, while the transit-selected sample is similarly active. Given the factor of 2-3 scatter in fractional FUV luminosity for a given stellar effective temperature, it is necessary to conduct a time-resolved study of the planet hosts in order to better characterize their UV variability and generate a firmer statistical result. © 2013. The American Astronomical Society. All rights reserved..


We here introduce a simple nonlinear model to describe the rotational evolution of cool stars on the main sequence. It is formulated only in terms of the Rossby number (Ro = P/τ), its inverse, and two dimensionless constants which we specify using solar and open-cluster data. The model has two limiting cases of stellar rotation, previously called C and I, that correspond to two observed sequences of fast and slowly rotating stars in young open clusters. The model describes the evolution of stars from C-type, with particular mass and age dependencies, to I-type, with different mass and age dependencies, through the rotational gap, g, separating them. The proposed model explains various aspects of stellar rotation, and provides an exact expression for the age of a rotating cool star in terms of P and t , thereby generalizing gyrochronology. Using it, we calculate the time interval required for stars to reach the rotational gap-a monotonically increasing, mildly nonlinear function of t . Beginning with the range of initial periods indicated by observations, we show that the (mass-dependent) dispersion in rotation period initially increases, and then decreases rapidly with the passage of time. The initial dispersion in period contributes up to 128 Myr to the gyro-age errors of solar-mass field stars. Finally, we transform to color-period space, calculate appropriate isochrones, and show that this model explains some detailed features in the observed color-period diagrams of open clusters, including the positions and shapes of the sequences, and the observed density of stars across these diagrams. © 2010. The American Astronomical Society. All rights reserved.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: STELLAR ASTRONOMY & ASTROPHYSC | Award Amount: 362.70K | Year: 2015

Although Earth orbits a single star, most stars in our Galaxy are located in binary or multiple systems - groups of two or more stars that are gravitationally bound to each other. The time it takes the two stars in a binary to complete one full orbit around each other can be anywhere from hours to days, weeks, month, years, or millennia. The stars in the closest pairs with orbits that take months or less, comprise a special category called spectroscopic binaries. They move so rapidly that it is possible to measure the velocities of each star, and from the velocities we can determine the ratio of their masses. Among very young, newly formed stars, these spectroscopic binaries provide a way to measure the most fundamental stellar property, mass, during a poorly understood period in the life-cycle of stars. This can help us calibrate the relationships between fundamental stellar properties at a key epoch: the onset of planet formation. In
this program, the PI will observe a sample of 140 candidate young spectroscopic binaries to confirm their
multiplicity and to measure the mass ratios of the bona fide pairs. This research will be carried out by the PI in collaboration with undergraduate and junior graduate students, some of them from groups traditionally underrepresented in astronomy.

Measuring the mass ratios and masses of young stars is important for understanding the physical processes of star and binary formation. Usually, this is accomplished by comparing the locations of stars on an H-R diagram to theoretical tracks of young star evolution. However, the wide variety of available calculations predict a large range of possible stellar masses at young ages. The PIs work in this area has helped to renew interest in improving these models. In this program, she will observe a sample of 140 candidate young spectroscopic binaries to confirm their multiplicity with multi-object visible light spectroscopy on the MMT Telescope and to measure the mass ratios of the bona fide pairs with 3 epochs of infrared spectroscopy using the Discovery Channel Telescope.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: STELLAR ASTRONOMY & ASTROPHYSC | Award Amount: 359.24K | Year: 2013

This is an observational project devoted to measuring stellar and disk properties for a hundred young multiple star systems using high-resolution spectroscopy and imaging techniques at infrared wavelengths, mainly using Keck Observatory. She will use the data to determine effective temperatures, radial velocities, rotation rates and surface gravities of stars, as well as accretion rates and magnetic fields associated with surrounding disk material. The data will be used to test models of binary formation and angular momentum distribution of planetary systems and circumstellar disks.

Broader impacts include training a postdoctoral researcher, and updating a website with a database on visual binary stars for the community. The PI also plans to continue giving astronomy presentations to local senior centers.


Grant
Agency: NSF | Branch: Continuing grant | Program: | Phase: CAREER: FACULTY EARLY CAR DEV | Award Amount: 268.89K | Year: 2014

This award will allow Dr. Kevin Covey to perform the INfrared Survey of Young Nebulous Clusters (IN-SYNC), which is a homogeneous, multi-epoch radial velocity survey of thousands of young stars in nearby clusters. IN-SYNC uses data from the Apachee Point Observatory Galactic Evolution Experiment (APOGEE), as well as the Immersion Grating INfrared Spectrometer (IGRINS), in order to answer three questions:

1) How do star clusters form?
2) What is the frequency of close binaries in young clusters?
3) What are the detailed stellar properties of pre-main sequence spectroscopic binaries?

The PI will also integrate his research and education programs by engaging citizen scientists through the Lowell Amateur Research Initiative (LARI). He will initially work with amateur astronomers to monitor variable young stars, which will provide valuable data for peer-reviewed publications. Dr. Covey will subsequently expand his program by directly connecting LARI participants to data from the robotic telescopes from Lowell Observatory.

Since star clusters are invaluable astrophysical laboratories, this work will provide unique tracers of the Milky Ways chemical evolution and star formation history, as well as enable stringent tests of stellar evolution models. Through the LARI initiative, this work will also result in increased public scientific literacy, as well as inform a new generation of citizen-science projects.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: GALACTIC ASTRONOMY PROGRAM | Award Amount: 579.52K | Year: 2016

Massive stars are the cosmic engines of the Universe. These stars shine because of nuclear reactions in their cores. This process also has made most of the oxygen and carbon atoms that exist today. When these massive stars die, lighter elements are turned into heavier ones, such as iron. We are literally made up of this star stuff. These stars also give off so much energy that they cause new stars to form. Although we understand most kinds of stars pretty well, there are many things about massive stars that remain a mystery. The goal of this project is to see how well our theories work by comparing predictions against observations. This project will give us a clearer picture of what happens as a massive star ages. The investigators will compare the type of stars found in different nearby galaxies, using data taken on large telescopes in Arizona and in Chile. This project will also fund two early-career research students, and provide research projects for two undergraduates.

The investigators will study massive stars in several ways. They plan to observe Local Group galaxies, which are close enough to be studied in great detail. They will observe all massive stars in these galaxies. Specifically they will determine 1) how changing orbits of binary stars modify the element formation in massive stars and 2) how the most massive stars end their lives.

The investigator and his collaborators have begun a survey for Wolf-Rayet (WR) stars in the Magellanic Clouds. With half of the area covered, the project has already confirmed the existence of 15 new WR stars, 10 of which appear to be a new type, never before seen. These discoveries suggest that the population of WR stars in the Magellanic Clouds may have been underestimated by as much as 40%, and yet for decades the Magellanic Clouds have served as the linchpins for evaluating massive star models at low metallicities. When complete, the Magellanic Cloud WR Survey will permit the determination of what fraction of WR stars have changed through binary interactions (Roche-lobe overflow) and fraction show changes in element abundance through single-star material losses due to stellar winds. Thirty WR binaries have been identified in M31 and M33, and repeated radial velocity measurements are being made with the 6.5-m MMT telescope, along with photometric monitoring at Lowell Observatory?s new 4.3-m telescope. This project will provide similar data on the frequency of binary interactions at higher metallicity, and provide direct measurements of the masses of WR stars for comparison with model predictions. The nature of the mysterious, oxygen-rich, WR stars (WO stars) will be explored by determining accurate surface abundances of carbon and oxygen. Comparing these observations to models will determine if the WO stars are truly the last hurrah of the most massive stars. Each of these experiments provides a key piece of information to improve models of massive star nuclear energy production.

The investigator will bring undergraduates into his research program, providing a boost to students from small, liberal arts colleges. This experience enables their students to successfully compete for entrance to best graduate schools in the country. These students are drawn from the REU program administered through Northern Arizona University, as well as Lowell Observatorys MIT Field Camp, which now includes students from the all-women?s Wellesley College.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: STELLAR ASTRONOMY & ASTROPHYSC | Award Amount: 46.16K | Year: 2014

Stars many times more massive than our Sun are rare, but they are responsible for producing nearly all of the heavier elements found in nature. Through dramatic supernova explosions, they produce and shed this material throughout the galaxy, mixing it with material out of which the next generation of stars, and planets, are formed. One interesting class of stars, known as Be stars, rotate so rapidly that they somehow shed material that then forms in a disk around their equators. In order to better understand how these disks are formed and why they occasionally seem to disappear, researchers at the University of Oklahoma, Lowell Observatory, and the University of Toledo have formed a team to carry out an intensive study of these systems. They will use an array of ground-based telescopes to observe these events, and they are developing sophisticated computer codes to aid the interpretation of these observations. This program will support graduate students at the University of Oklahoma and at the University of Toledo, as well as undergraduate students at all three institutions. They will develop research-based planetarium shows that satisfy state education standards. Undergraduates will be trained to develop content and conduct outreach and to take these shows on the road to K-12 students using a portable planetarium and a digital projector. These shows will be freely released for use in other settings.

This program will determine the timescales, relative prevalence, and physical mechanisms of disk-loss and disk rebuilding episodes of classical Be stars. These goals will be accomplished by carrying out a statistical assessment of the prevalence of disk-loss/renewal episodes of populations of Be stars in open clusters, as well as intensive multi wavelength studies of individual Be stars actively experiencing disk-loss/renewal episodes. Predictions of state-of-the-art radiation transfer and hydrodynamical modeling codes will be compared against the data obtained in this program, to test a range of scenarios that have been proposed to explain how circumstellar disks may be created and destroyed during the lifetime of the host star.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: STELLAR ASTRONOMY & ASTROPHYSC | Award Amount: 564.91K | Year: 2016

This collaborative project will use imaging and spectrophotometry to study 2,000 of the nearest stars. The high spatial resolution images will be capable of revealing bound planets. The spectrophotometry will provide information about the systems? temperatures and orbital characteristics. The work will focus on a type of star called M-dwarfs, which are cooler and less massive than the Sun. These stars are also considered more likely to have rocky planets habitable to life than stars like the Sun. Observations will be made with two telescopes in Arizona and one in California. Scientific results will be incorporated into an outreach program with the Discovery Channel. Students from Northern Arizona University and Southern Connecticut State University will participate in the research.

The team will use the Differential Speckle Survey Instrument (DSSI) on Lowell Observatory 4.5m telescope and the R=600 visible-light multi-object spectrophotometer on Lowell?s 31-inch telescope. Through the comparison of speckle patterns simultaneously observed at two wavelengths, DSSI can measure separations of companions below the diffraction limit. Follow-up spectrophotometry will provide data for stellar characterization and spectral energy distribution fitting. The team will also use the Palomar 200-inch telescope to assess bound companions using data from an infrared camera or optical integral field spectrometer and isochrone modeling.

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