arXiv subjects

M. R. Meyer

Publications and source records attributed to M. R. Meyer.

70 records · Page 4Linked to original sources

arXiv2006

Observations of Herbig Ae Disks with Nulling Interferometry

We present the results of 10 micron nulling interferometric observations of 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 m telescopes. A portion of the observations was completed with the adaptive secondary at the MMT. We have conclusively spatially resolved 3 of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter two recently resolved using adaptive optics in combination with nulling interferometry. For the resolved objects we find that the 10 $μ$m emitting regions have a spatial extent of 15-30 AU in diameter. We also have some evidence for resolved emission surrounding an additional 2 stars (V892 Tau and R CrA). For those objects in our study with mid-IR SEDs classifications from Meeus et al. (2001), we find that the Group I objects (those with constant to increasing mid-IR flux) are more likely to be resolved, within our limited sample. This trend is evident in correlations in the inferred disk sizes vs. the sub-millimeter SED slope and disk size vs. fractional infrared luminosity of the systems. We explore the spatial distribution and orientation of the warm dust in the resolved systems and constrain physical models which are consistent with their observational signatures.

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arXiv2006

NICMOS/HST Observations of the Embedded Cluster Associated with Mon R2: Constraining the sub-stellar Initial Mass Function

We have analyzed HST/NICMOS2 F110W, F160W, F165M, and F207M band images covering the central 1'x1' of the cluster associated with Mon R2 in order to constrain the Initial Mass Function (IMF) down to 20 Mjup. The flux ratio between the F165M and F160W bands was used to measure the strength of the water band absorption feature and select a sample of 12 out of the total sample of 181 objects that have effective temperatures between 2700 K and 3300 K. These objects are placed in the HR diagram together with sources observed by Carpenter et al. (1997) to estimate an age of ~1 Myr for the low mass cluster population. By constructing extinction limited samples, we are able to constrain the IMF and the fraction of stars with a circumstellar disk in a sample that is 90% complete for both high and low mass objects. For stars with estimated masses between 0.1 Msun and 1.0 Msun for a 1 Myr population with Av < 19 mag, we find that 27+-9% have a near-infrared excess indicative of a circumstellar disk. The derived fraction is similar to, or slightly lower than, the fraction found in other star forming regions of comparable age. We constrain the number of stars in the mass interval 0.08-1.0Msun to the number of objects in the mass interval 0.02-0.08 Msun by forming the ratio, R**=N(0.08-1Msun)/N(0.02-0.08Msun) for objects in an extinction limited sample complete for Av < 7 mag. The ratio is found to be R^**=2.2+-1.3 assuming an age of 1 Myr, consistent with the similar ratio predicted by the system IMF proposed by Chabrier (2003). The ratio is similar to the ratios observed towards the Orion Nebula Cluster and IC 348 as well as the ratio derived in the 28 square degree survey of Taurus by Guieu et al. (2006).

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arXiv2006

Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in Disks Around Sun-like Stars

We have carried out a sensitive search for gas emission lines at infrared and millimeter wavelengths for a sample of 15 young sun-like stars selected from our dust disk survey with the Spitzer Space Telescope. We have used mid-infrared lines to trace the warm (300-100 K) gas in the inner disk and millimeter transitions of 12CO to probe the cold (~20 K) outer disk. We report no gas line detections from our sample. Line flux upper limits are first converted to warm and cold gas mass limits using simple approximations allowing a direct comparison with values from the literature. We also present results from more sophisticated models following Gorti and Hollenbach (2004) which confirm and extend our simple analysis. These models show that the SI line at 25.23 micron can set constraining limits on the gas surface density at the disk inner radius and traces disk regions up to a few AU. We find that none of the 15 systems have more than 0.04 MJ of gas within a few AU from the disk inner radius for disk radii from 1 AU up to ~40 AU. These gas mass upper limits even in the 8 systems younger than ~30 Myr suggest that most of the gas is dispersed early. The gas mass upper limits in the 10-40 AU region, that is mainly traced by our CO data, are <2 Mearth. If these systems are analogs of the Solar System, either they have already formed Uranus- and Neptune-like planets or they will not form them beyond 100 Myr. Finally, the gas surface density upper limits at 1 AU are smaller than 0.01% of the minimum mass solar nebula for most of the sources. If terrestrial planets form frequently and their orbits are circularized by gas, then circularization occurs early.

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arXiv2006

Evolution of Circumstellar Disks Around Normal Stars: Placing Our Solar System in Context

Over the past 10 years abundant evidence has emerged that many (if not all) stars are born with circumstellar disks. Understanding the evolution of post-accretion disks can provide strong constraints on theories of planet formation and evolution. In this review, we focus on developments in understanding: a) the evolution of the gas and dust content of circumstellar disks based on observational surveys, highlighting new results from the Spitzer Space Telescope; b) the physical properties of specific systems as a means to interpret the survey results; c) theoretical models used to explain the observations; d) an evolutionary model of our own solar system for comparison to the observations of debris disks around other stars; and e) how these new results impact our assessment of whether systems like our own are common or rare compared to the ensemble of normal stars in the disk of the Milky Way.

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arXiv2006

Erratum: "Post-T Tauri Stars in the Nearest OB Association" (AJ, 124, 1670 [2002])

A few typos in Mamajek, Meyer, & Liebert (2002, AJ, 124, 1670) have been corrected in this erratum (including two stellar misidentifications and an incorrect power in the units of a slope). The most significant is the correction of a sign error in the published polynomial conversion between Tycho and Johnson-Cousins (B-V) colors.

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arXiv2005

Formation and Evolution of Planetary Systems (FEPS): Primordial Warm Dust Evolution From 3-30 Myr around Sun-like Stars

We present data obtained with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope (Spitzer) for a sample of 74 young (t < 30 Myr old) Sun-like (0.7 < M(star)/M(Sun) < 1.5) stars. These are a sub-set of the observations that comprise the Spitzer Legacy science program entitled the Formation and Evolution of Planetary Systems (FEPS). Using IRAC we study the fraction of young stars that exhibit 3.6-8.0 micron infrared emission in excess of that expected from the stellar photosphere, as a function of age from 3-30 Myr. The most straightforward interpretation of such excess emission is the presence of hot (300-1000K) dust in the inner regions (< 3 AU) of a circumstellar disk. Five out of the 74 young stars show a strong infrared excess, four of which have estimated ages of 3-10 Myr. While we detect excesses from 5 optically thick disks, and photospheric emission from the remainder of our sample, we do not detect any excess emission from optically thin disks at these wavelengths. We compare our results with accretion disk fractions detected in previous studies, and use the ensemble results to place additional constraints on the dissipation timescales for optically-thick, primordial disks.

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arXiv2005

Bispectrum speckle interferometry of the massive protostellar outflow source IRAS 23151+5912

We present bispectrum speckle interferometry of the massive protostellar object IRAS 23151+5912 in the near-infrared K' band. The reconstructed image shows the diffuse nebulosity north-east of two point-like sources in unprecedented detail. The comparison of our near-infrared image with mm continuum and CO molecular line maps shows that the brighter of the two point sources lies near the center of the mm peak, indicating that it is a high-mass protostar. The nebulosity coincides with the blue-shifted molecular outflow component. The most prominent feature in the nebulosity is a bow-shock-like arc. We assume that this feature is associated with a precessing jet which has created an inward-pointed cone in the swept-up material. We present numerical jet simulations that reproduce this and several other features observed in our speckle image of the nebulosity. Our data also reveal a linear structure connecting the central point source to the extended diffuse nebulosity. This feature may represent the innermost part of a jet that drives the strong molecular outflow (PA ~80 degr) from IRAS 23151+5912. With the aid of radiative transfer calculations, we demonstrate that, in general, the observed inner structures of the circumstellar material surrounding high-mass stars are strongly influenced by the orientation and symmetry of the bipolar cavity.

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arXiv2005

The Formation and Evolution of Planetary Systems (FEPS): Discovery of an Unusual Debris System Associated with HD 12039

We report the discovery of a debris system associated with the $\sim 30$ Myr old G3/5V star HD 12039 using {\it Spitzer Space Telescope} observations from 3.6 -- 160$μ$m. An observed infrared excess (L$_{\rm IR}$/L$_{\ast} = 1\times10^{-4}$) above the expected photosphere for $λ\gtrsim 14μ$m is fit by thermally emitting material with a color temperature of T$\sim 110$ K, warmer than the majority of debris disks identified to date around Sun-like stars. The object is not detected at 70$μ$m with a 3$σ$ upper limit 6 times the expected photospheric flux. The spectrum of the infrared excess can be explained by warm, optically thin material comprised of blackbody-like grains of size $\gtrsim 7 μ$m that reside in a belt orbiting the star at 4--6 AU. An alternate model dominated by smaller grains, near the blow-out size $a\sim 0.5μ$m, located at 30-40AU is also possible, but requires the dust to have been produced recently since such small grains will be expelled from the system by radiation pressure in $\sim$ few $\times 10^{2}$yrs.

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arXiv2005

Spitzer Observations of G Dwarfs in the Pleiades: Circumstellar Debris Disks at 100 Myr Age

Fluxes and upper limits in the wavelength range from 3.6 to 70 microns from the Spitzer Space Telescope are provided for twenty solar-mass Pleiades members. One of these stars shows a probable mid-IR excess and two others have possible excesses, presumably due to circumstellar debris disks. For the star with the largest, most secure excess flux at MIPS wavelengths, HII1101, we derive Log(L[dust]/L[Sun]) ~ -3.8 and an estimated debris disk mass of 4.2 x 10^-5 M(Earth) for an assumed uniform dust grain size of 10 microns If the stars with detected excesses are interpreted as stars with relatively recent, large collision events producing a transient excess of small dust particles, the frequency of such disk transients is about ~ 10 % for our ~ 100 Myr, Pleiades G dwarf sample. For the stars without detected 24-70 micron excesses, the upper limits to their fluxes correspond to approximate 3 sigma upper limits to their disk masses of 6 x 10^-6 M(Earth) using the MIPS 24 micron upper limit, or 2 x 10^-4 M(Earth) using the MIPS 70 micron limit. These upper limit disk masses (for "warm" and "cold" dust, respectively) are roughly consistent, but somewhat lower than, predictions of a heuristic model for the evolution of an "average" solar-mass star's debris disk based on extrapolation backwards in time from current properties of the Sun's Kuiper belt.

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arXiv2005

The IMF in Extreme Star-Forming Environments: Searching for Variations vs. Initial Conditions

Any predictive theory of star formation must explain observed variations (or lack thereof) in the initial mass function. Recent work suggests that we might expect quantitative variations in the IMF as a function of metallicity (Larson 2005) or magnetic field strength (Shu et al. 2004). We summarize results from several on-going studies attempting to constrain the ratio of high to low mass stars, as well as stars to sub- stellar objects, in a variety of different environments, all containing high mass stars. First, we examine the ratio of stars to sub--stellar objects in the nearby Mon R2 region utilizing NICMOS/HST data. We compare our results to the IMF by Kroupa (2002)]} and to the observed ratios for IC 348 and Orion. Second, we present preliminary results for the ratio of high to low mass stars in W51, the most luminous HII region in the galaxy. Based on ground--based multi--colour images of the cluster obtained with the MMT adaptive optics system, we derive a lower limit to the ratio of high-mass to low-mass stars and compare it to the ratios for nearby clusters. Finally, we present the derived IMF for the R136 region in the LMC where the metallicity is 1/4 solar using HST/NICMOS data. We find that the IMF is consistent with that characterizing the field (Chabrier 2003), as well as nearby star--forming regions, down to 1.0 Msun outside 2 pc. Whereas the results for both Mon R2 and R136 are consistent with the nearby clusters, the ratio of high to low mass stars in W51 tentatively indicates a lack of low--mass objects.

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arXiv2004

8--13 um spectroscopy of YSOs: Evolution of the silicate feature

In order to investigate possible connections between dust processing and disk properties, 8--13 um spectra of 34 young stars, with a range of circumstellar environments and spectral types A to M, were obtained using the Long Wavelength Spectrometer at the W. M. Keck Observatory. The broad 9.7 um amorphous silicate feature which dominates this wavelength regime evolves from absorption in young, embedded sources, to emission in optically revealed stars, and to complete absence in older debris disk systems for both low- and intermediate-mass stars. The peak wavelength and FWHM are centered about 9.7 and ~2.3 um, corresponding to amorphous olivine, with a larger spread in FWHM for embedded sources and in peak wavelength for disks. In a few of our objects that have been previously identified as class I low-mass YSOs, the observed silicate feature is complex, with absorption near 9.5 um and emission peaking around 10 um. Although most of the emission spectra show broad classical features attributed to amorphous silicates, variations in the shape/strength may be linked to dust processing, including grain growth and/or silicate crystallization. We study quantitatively the evidence for evolutionary trends in the 8--13 um spectra through a variety of spectral shape diagnostics. Based on the lack of correlation between these diagnostics and broad-band infrared luminosity characteristics for silicate emission sources, we conclude that although spectral signatures of dust processing are present, they can not be connected clearly to disk evolutionary stage (for optically thick disks) or optical depth (for optically thin disks). The diagnostics of silicate absorption features (other than the central wavelength of the feature), however, are tightly correlated with optical depth.

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arXiv2004

Gould's Belt to Starburst Galaxies: The IMF of Extreme Star Formation

Recent results indicate the stellar initial mass function is not a strong function of star-forming environment or ``initial conditions'' (e.g. Meyer et al. 2000). Some studies suggest that a universal IMF may extend to sub-stellar masses (see however Briceno et al. 2002). Yet most of this work is confined to star-forming environments within 1 kpc of the Sun. In order to probe the universality of the IMF over a wider range of parameter space (metalicity, ambient pressure, magnetic field strength) new techniques are required. We begin by summarizing our approach to deriving the sub-stellar IMF down to the opacity-limit for fragmentation using NGC 1333 as an example. Next, we describe results from simulations using the observed point-spread function of the new 6.5m MMT adaptive optics system and examine the confusion-limited sensitivity to low mass stars in rich star-forming clusters out to 0.5 Mpc. We also present preliminary results from observations with this system of the W51 star-forming complex. Finally, we outline a new technique to estimate the ratio of high to low mass stars in unresolved stellar populations, such as the massive star clusters observed in interacting galaxies (e.g. Mengel et al. 2002). While evidence for variations in the IMF remains inconclusive, new studies are required to rule them out and determine whether or not the IMF is universal over the range of parameter space relevant to star-forming galaxies over cosmic time.

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arXiv2004

The Formation and Evolution of Planetary Systems: First Results from a Spitzer Legacy Science Program

We present 3-160 micron photometry obtained with the IRAC and MIPS instruments for the first five targets from the Spitzer Legacy Science Program "Formation and Evolution of Planetary Systems" and 4-35 micron spectro-photometry obtained with the IRS for two sources. We discuss in detail our observations of the debris disks surrounding HD 105 (G0V, 30 +- 10 Myr) and HD 150706 (G3V, ~ 700 +- 300 Myr). For HD 105, possible interpretations include large bodies clearing the dust inside of 45 AU or a reservoir of gas capable of sculpting the dust distribution. The disk surrounding HD 150706 also exhibits evidence of a large inner hole in its dust distribution. Of the four survey targets without previously detected IR excess, spanning ages 30 Myr to 3 Gyr, the new detection of excess in just one system of intermediate age suggests a variety of initial conditions or divergent evolutionary paths for debris disk systems orbiting solar-type stars.

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arXiv2003

A Resolved Circumstellar Disk around the Herbig Ae Star HD 100546 in the Thermal Infrared

We present mid-infrared nulling interferometric and direct imaging observations of the Herbig Ae star HD 100546 obtained with the Magellan I (Baade) 6.5 m telescope. The observations show resolved circumstellar emission at 10.3, 11.7, 12.5, 18.0, and 24.5 microns. Through the nulling observations (10.3, 11.7 and 12.5 microns), we detect a circumstellar disk, with an inclination of 45 +- 15 degrees with respect to a face-on disk, a semimajor axis position angle of 150 +- 10 degrees (E of N), and a spatial extent of about 25 AU. The direct images (18.0 and 24.5 microns) show evidence for cooler dust with a spatial extent of 30-40 AU from the star. The direct images also show evidence for an inclined disk with a similar position angle as the disk detected by nulling. This morphology is consistent with models in which a flared circumstellar disk dominates the emission. However, the similarity in relative disk size we derive for different wavelengths suggests that the disk may have a large inner gap, possibly cleared out by the formation of a giant protoplanet. The existence of a protoplanet in the system also provides a natural explanation for the observed difference between HD 100546 and other Herbig Ae stars.

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arXiv2001

The Formation and Evolution of Planetary Systems: SIRTF Legacy Science in the VLT Era

We will utilize the sensitivity of SIRTF through the Legacy Science Program to carry out spectrophotometric observations of solar-type stars aimed at (1) defining the timescales over which terrestrial and gas giant planets are built, from measurements diagnostic of dust/gas masses and radial distributions; and (2) establishing the diversity of planetary architectures and the frequency of planetesimal collisions as a function of time through observations of circumstellar debris disks. Together, these observations will provide an astronomical context for understanding whether our solar system - and its habitable planet - is a common or a rare circumstance. Achieving our science goals requires measuring precise spectral energy distributions for a statistically robust sample capable of revealing evolutionary trends and the diversity of system outcomes. Our targets have been selected from two carefully assembled databases of solar-like stars: (1) a sample located within 50 pc of the Sun spanning an age range from 100-3000 Myr for which a rich set of ancillary measurements (e.g. metallicity, stellar activity, kinematics) are available; and (2) a selection located between 15 and 180 pc and spanning ages from 3 to 100 Myr. For stars at these distances SIRTF is capable of detecting stellar photospheres with SNR >30 at lambda < 24 microns for our entire sample, as well as achieving SNR >5 at the photospheric limit for over 50% of our sample at lambda=70 microns. Thus we will provide a complete census of stars with excess emission down to the level produced by the dust in our present-day solar system. More information concerning our program can be found at: http://gould.as.arizona.edu/feps

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arXiv1997

Circumstellar Disks Around Pre-Main-Sequence Stars: What ISO Can Tell Us

ISO observations will improve our understanding of disks around low-mass pre-main-sequence stars in various ways. In particular, ISO can measure simultaneously spectral energy distributions and low-resolution spectra from 2 to 12 \um of T Tauri stars with greater sensitivity, greater spectral coverage and finer spectral resolution than available in ground-based observations or with IRAS. We illustrate the importance of such observations for our understanding of the disk structure and heating mechanisms, the disk evolution (in particular the formation of gaps caused by the presence of companion stars or large planets) and dissipation using preliminary results from PHOT and PHOT-S for T Tauri stars in the Chamaeleon cloud.

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