703 research outputs found
Snacks 4 the Brain--Keivan Stassun!
No full textIncludes descriptive metadata provided by producer in MP3 file: "Snacks 4 the Brain! - Podcasts - Snacks 4 the Brain--Keivan Stassun!" Scott Merrick interviews Keivan Stassun, an member of the Vanderbilt Physics and Astronomy Department who also works to help increase, through K-12 education, the representation of minority groups among astronomers. He discusses this as well as his own development as an astronomer.Vanderbilt University. Medical Cente
VERY LOW MASS STELLAR AND SUBSTELLAR COMPANIONS TO SOLAR-LIKE STARS FROM MARVELS. I. A LOW-MASS RATIO STELLAR COMPANION TO TYC 4110-01037-1 IN A 79 DAY ORBIT
No full textTYC 4110-01037-1 has a low-mass stellar companion, whose small mass ratio and short orbital period are atypical among binary systems with solar-like (T [subscript eff] [< over ~] 6000 K) primary stars. Our analysis of TYC 4110-01037-1 reveals it to be a moderately aged ([< over ~] Gyr) solar-like star having a mass of 1.07 ± 0.08 M [subscript ☉] and radius of 0.99 ± 0.18 R [subscript ☉]. We analyze 32 radial velocity (RV) measurements from the SDSS-III MARVELS survey as well as 6 supporting RV measurements from the SARG spectrograph on the 3.6 m Telescopio Nazionale Galileo telescope obtained over a period of ~2 years. The best Keplerian orbital fit parameters were found to have a period of 78.994 ± 0.012 days, an eccentricity of 0.1095 ± 0.0023, and a semi-amplitude of 4199 ± 11 m s[superscript –1]. We determine the minimum companion mass (if sin i = 1) to be 97.7 ± 5.8 M [subscript Jup]. The system's companion to host star mass ratio, ≥0.087 ± 0.003, places it at the lowest end of observed values for short period stellar companions to solar-like (T [subscript eff] [< over ~] 6000 K) stars. One possible way to create such a system would be if a triple-component stellar multiple broke up into a short period, low q binary during the cluster dispersal phase of its lifetime. A candidate tertiary body has been identified in the system via single-epoch, high contrast imagery. If this object is confirmed to be comoving, we estimate it would be a dM4 star. We present these results in the context of our larger-scale effort to constrain the statistics of low-mass stellar and brown dwarf companions to FGK-type stars via the MARVELS survey.Vanderbilt University (Initiative in Data-Intensive Astrophysics)National Science Foundation (U.S.) (CAREER Grant AST0349075
THE INITIAL MASS FUNCTION OF THE ORION NEBULA CLUSTER ACROSS THE H-BURNING LIMIT
No full textWe present a new census of the Orion Nebula Cluster over a large field of view (gsim 30' × 30'), significantly increasing the known population of stellar and substellar cluster members with precisely determined properties. We develop and exploit a technique to determine stellar effective temperatures from optical colors, nearly doubling the previously available number of objects with effective temperature determinations in this benchmark cluster. Our technique utilizes colors from deep photometry in the I band and in two medium-band filters at λ ~ 753 and 770 nm, which accurately measure the depth of a molecular feature present in the spectra of cool stars. From these colors we can derive effective temperatures with a precision corresponding to better than one-half spectral subtype, and importantly this precision is independent of the extinction to the individual stars. Also, because this technique utilizes only photometry redward of 750 nm, the results are only mildly sensitive to optical veiling produced by accretion. Completing our census with previously available data, we place some 1750 sources in the Hertzsprung-Russell diagram and assign masses and ages down to 0.02 solar masses. At faint luminosities, we detect a large population of background sources which is easily separated in our photometry from the bona fide cluster members. The resulting initial mass function of the cluster has good completeness well into the substellar mass range, and we find that it declines steeply with decreasing mass. This suggests a deficiency of newly formed brown dwarfs in the cluster compared to the Galactic disk population.Deutsche Forschungsgemeinschaft (Sonderforschungsbereich SFB 881 “The Milky Way System” (subproject B6)
Detection of strong activity in the eclipsing binary brown dwarf 2MASS J05352184-0546085: a possible explanation for the temperature reversal
No full textWe show high-resolution spectra of the eclipsing brown dwarf binary 2MASS J05352184 - 0546085 taken at the two opposite radial velocity maxima. Comparisons of the TiO bands to model and template spectra are fully consistent with the temperatures previously derived for this system. In particular, the reversal of temperatures with mass - in which the higher mass primary is cooler than its companion - is confirmed. We measure the projected rotation velocities of the components; the primary is rotating at least twice as rapidly as the secondary. At the two radial velocity maxima, H alpha emission lines of both components stick out to either sides of the H alpha central wavelength, which is dominated by nebula emission. This enables us to model the individual H alpha lines of the primary and the secondary. We find that the H alpha emission from the primary is at least 7 times stronger than the emission from the secondary. We conclude that the temperature reversal is very likely due to strong magnetic fields inhibiting convection on the primary
HUBBLE SPACE TELESCOPE MEASURES OF MASS ACCRETION RATES IN THE ORION NEBULA CLUSTER
No full textThe present observational understanding of the evolution of the mass accretion rates ([dot over M][subscript acc]) in pre-main-sequence stars is limited by the lack of accurate measurements of [dot over M][subscript acc] over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on Hubble Space Telescope/WFPC2 observations, we present a study of [dot over M][subscript acc] for a sample of ~700 sources in the Orion Nebula Cluster, ranging from the hydrogen-burning limit to M [subscript *] ~ 2 M [subscript ☉]. We derive [dot over M][subscript acc] from both the U-band excess and the Hα luminosity (L [subscript Hα]), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (L [subscript acc]) and L [subscript Hα], which is L [subscript acc]/L [subscript ☉] = (1.31 ± 0.03) · L [subscript Hα]/L [subscript ☉] + (2.63 ± 0.13). Given our large statistical sample, we are able to accurately investigate relations between [dot over M][subscript acc] and the parameters of the central star such as mass and age. We clearly find [dot over M][subscript acc] to increase with stellar mass and decrease over evolutionary time, but we also find strong evidence that the decay of [dot over M][subscript acc] with stellar age occurs over longer timescales for more massive PMS stars. Our best-fit relation between these parameters is given by log ([dot over M][subscript acc]/M [subscript ☉] yr) = (–5.12 ± 0.86) – (0.46 ± 0.13) · log (t/yr) – (5.75 ± 1.47) · log (M [subscript *]/M [subscript ☉]) + (1.17 ± 0.23) · log (t/yr) · log (M [subscript *]/M [subscript ☉]). These results also suggest that the similarity solution model could be revised for sources with M [subscript *] [> over ~] 0.5 M [subscript ☉]. Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources
Bright X-Ray Flares in Orion Young Stars from COUP: Evidence for Star-Disk Magnetic Fields?
No full textWe have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 day observation of the Orion Nebula Cluster (ONC), concentrating on the events with the highest statistics (in terms of photon flux and event duration). Analysis of the flare decay allows to determine the physical parameters of the flaring structure, particularly its size and (using the peak temperature and emission measure of the event) the peak density, pressure, and minimum confining magnetic field. A total of 32 events, representing the most powerful ~=1% of COUP flares, have sufficient statistics and are sufficiently well resolved to grant a detailed analysis. A broad range of decay times are present in the sample of flares, with τlc (the 1/e decay time) ranging from 10 to 400 ks. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures that are found, from the analysis of the flare's decay, to confine the plasma are in a number of cases very long, with semilengths up to ~=1012 cm, implying the presence of magnetic fields of hundreds of G (necessary to confine the hot flaring plasma) extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures (length L>>R*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L<=R*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures that confine the flaring plasma are actually the same type of structures that channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk
LP 714-47 b (TOI 442.01): populating the Neptune desert
No full textWe report the discovery of a Neptune-like planet (LP 714-47 b, P = 4.05204 d, m b = 30.8 ± 1.5 M ⊕ , R b = 4.7 ± 0.3 R ⊕ ) located in the “hot Neptune desert”. Confirmation of the TESS Object of Interest (TOI 442.01) was achieved with radial-velocity follow-up using CARMENES, ESPRESSO, HIRES, iSHELL, and PFS, as well as from photometric data using TESS, Spitzer , and ground-based photometry from MuSCAT2, TRAPPIST-South, MONET-South, the George Mason University telescope, the Las Cumbres Observatory Global Telescope network, the El Sauce telescope, the TÜBİTAK National Observatory, the University of Louisville Manner Telescope, and WASP-South. We also present high-spatial resolution adaptive optics imaging with the Gemini Near-Infrared Imager. The low uncertainties in the mass and radius determination place LP 714-47 b among physically well-characterised planets, allowing for a meaningful comparison with planet structure models. The host star LP 714-47 is a slowly rotating early M dwarf ( T eff = 3950 ± 51 K) with a mass of 0.59 ± 0.02 M ⊙ and a radius of 0.58 ± 0.02 R ⊙ . From long-term photometric monitoring and spectroscopic activity indicators, we determine a stellar rotation period of about 33 d. The stellar activity is also manifested as correlated noise in the radial-velocity data. In the power spectrum of the radial-velocity data, we detect a second signal with a period of 16 days in addition to the four-day signal of the planet. This could be shown to be a harmonic of the stellar rotation period or the signal of a second planet. It may be possible to tell the difference once more TESS data and radial-velocity data are obtained
Ionization-induced star formation - IV. Triggering in bound clusters
No full textWe present a detailed study of star formation occurring in bound star-forming clouds under the influence of internal ionizing feedback from massive stars across a spectrum of cloud properties. We infer which objects are triggered by comparing our feedback simulations with control simulations in which no feedback was present. We find that feedback always results in a lower star formation efficiency and usually but not always results in a larger number of stars or clusters. Cluster mass functions are not strongly affected by feedback, but stellar mass functions are biased towards lower masses. Ionization also affects the geometrical distribution of stars in ways that are robust against projection effects, but may make the stellar associations more or less subclustered depending on the background cloud environment. We observe a prominent pillar in one simulation which is the remains of an accretion flow feeding the central ionizing cluster of its host cloud and suggest that this may be a general formation mechanism for pillars such as those observed in M16. We find that the association of stars with structures in the gas such as shells or pillars is a good but by no means foolproof indication that those stars have been triggered and we conclude overall that it is very difficult to deduce which objects have been induced to form and which formed spontaneously simply from observing the system at a single time.Peer reviewe
Measuring Radial Velocities of Low Mass Eclipsing Binaries
No full textDue to the complex nature of the spectra of low-mass M type stars, it
is difficult to determine their metallicities and temperatures
directly. By studying eclipsing binary pairs comprising one F, G, or
K type star with an M type star, we are able to use what we know about
the primary star to learn more about the secondary star. Measuring
the orbital reflex motion of the primary star, together with the
eclipse light curve of the M star as it transits the primary star,
allows us to determine the mass, radius, temperature, and metallicity
of the M star.
We studied 23 low mass eclipsing binaries (EBLMs) previously
discovered by SuperWASP photometry. We obtained spectra using the
Cerro Tololo Inter-American Observatory (CTIO) SMARTS 1.5-meter
echelle spectrograph between June 2009 and January 2011. Each EBLM
target was typically observed ~8 times over this time period. The
spectra were processed using standard astronomical software, and a
cross-correlation method was used to measure the radial velocity of
the target star at each observed epoch.
Radial velocities were successfully determined for 21 of the 23 EBLM
target objects. Orbital periods, radial velocity amplitudes, and
eccentricities for these EBLMs could be determined from these radial
velocities together with the preexisting light curves. Using these
values and by assuming a mass for the primary star, we will be able to
calculate the masses of the secondary M type star in each EBLM system.College of Arts and ScienceVanderbilt UniversityDepartment of Physics and Astronom
Spectroscopic and Adaptive Optics Observations of Protostellar Magnetospheres and Binary Star Evolution
No full textOverall, there has been growing progress in the astrophysical literature towards the characterization of young stellar evolution phenomenon. The use of sensitive, near-infrared observations continues to unravel these mysteries, with this dissertation providing insight into the short, active periods during stellar formation and early binary dynamics. In Chapter 2, we analyze our K-band spectroscopy of Class 0 protostars taken by the Keck I MOSFIRE instrument. This data reveals the properties of their near-stellar environment, finding clear signatures of active disk accretion in the form of CO overtone emission. In particular, we find evidence for the presence of an established magnetosphere in Class 0s (the earliest observable stage of young stellar evolution) from our observed Brackett gamma line profiles. In Chapter 3, we analyze adaptive optics near-infrared imaging data of spectroscopic binaries (SBs) taken by the Robo-AO imaging system at the Kitt Peak National Observatory. With this high-contrast data, we searched for nearby companions in our field of view and find further evidence for the relation originally found by Tokovinin et al. 2006; shorter period SBs are systematically more likely to be in a higher order system than longer period SBs. Collectively, our findings support the mechanisms of angular momentum transfer from three-body interactions during early evolution, ultimately resulting in tight binaries with tertiaries that widen from pre-main-sequence to field ages. We briefly discuss future work and ideas in Chapter 4
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