Carl Melis

A VLBI resolution of the Pleiades distance controversy

Distance score settled for Seven Sisters Most of us have seen the Pleiades star cluster in the night sky, one of the few groups of physically related stars that are separately visible to the naked eye. In spite of its proximity to us, its distance has been disputed. Melis et al. settle the controversy with astrometric measurements from radio interferometry that reveal a distance of 136.2 parsecs (see the Perspective by Girardi). Other methods yielded similar values, but the trusted astrometry satellite Hipparcos measured only 120.2 parsecs. The new result alleviates the concern that astronomers would need to adjust their stellar evolution models to align with the Hipparcos distance. Science, this issue p. 1029; see also p. 1001 A highly accurate and precise radio measurement overrules a contrary result from the Hipparcos satellite. [Also see Perspective by Girardi] Because of its proximity and its youth, the Pleiades open cluster of stars has been extensively studied and serves as a cornerstone for our understanding of the physical properties of young stars. This role is called into question by the “Pleiades distance controversy,” wherein the cluster distance of 120.2 ± 1.5 parsecs (pc) as measured by the optical space astrometry mission Hipparcos is significantly different from the distance of 133.5 ± 1.2 pc derived with other techniques. We present an absolute trigonometric parallax distance measurement to the Pleiades cluster that uses very long baseline radio interferometry (VLBI). This distance of 136.2 ± 1.2 pc is the most accurate and precise yet presented for the cluster and is incompatible with the Hipparcos distance determination. Our results cement existing astrophysical models for Pleiades-age stars.

The H i opacity of the intergalactic medium at redshifts 1.6 < z < 3.2

We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range 2.2 < z < 3.2. Our measurement of DA excludes absorption from metal lines or the Lya lines of Lyman-limit systems and damped Lya systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between 1.6 < z < 2.2, we find that the redshift evolution of DA is well described over 1.6 < z < 3.2 as A (1 +z) γ , where A = 0.0062 and y = 2.75. We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less HI absorption than expected at z = 3, implying that the ultraviolet background is ∼40 per cent higher than expected. Our data appears to be consistent with an H I ionization rate of r ∼ 1.4 × 10 -12 s -1 .

AN ALUMINUM/CALCIUM-RICH, IRON-POOR, WHITE DWARF STAR: EVIDENCE FOR AN EXTRASOLAR PLANETARY LITHOSPHERE?

The presence of elements heavier than helium in white dwarf atmospheres is often a signpost for the existence of rocky objects that currently or previously orbited these stars. We have measured the abundances of various elements in the hydrogen-atmosphere white dwarfs G149-28 and NLTT 43806. In comparison with other white dwarfs with atmospheres polluted by heavy elements, NLTT 43806 is substantially enriched in aluminum but relatively poor in iron. We compare the relative abundances of Al and eight other heavy elements seen in NLTT 43806 with the elemental composition of bulk Earth, with simulated extrasolar rocky planets, with solar system meteorites, with the atmospheric compositions of other polluted white dwarfs, and with the outer layers of the Moon and Earth. The best agreement is found with a model that involves accretion of a mixture of terrestrial crust and upper mantle material onto NLTT 43806. The implication is that NLTT 43806 is orbited by a differentiated rocky planet, perhaps quite similar to Earth, that has suffered a collision that stripped away some of its outer layers.

THE AGE OF THE HD 15407 SYSTEM AND THE EPOCH OF FINAL CATASTROPHIC MASS ACCRETION ONTO TERRESTRIAL PLANETS AROUND SUN-LIKE STARS

From optical spectroscopic measurements we determine that the HD 15407 binary system is ~80?Myr old. The primary star, HD 15407A (spectral type F5 V), exhibits strong mid-infrared excess emission indicative of a recent catastrophic collision between rocky planetary embryos or planets in its inner planetary system. The synthesis of all known stars with large quantities of dust in their terrestrial planet zone indicates that for stars of roughly solar mass this warm dust phenomenon occurs at ages between 30 and 100?Myr. In contrast, for stars of a few solar masses, the dominant era of the final assembling of rocky planets occurs earlier, between 10 and 30?Myr age. The incidence of the warm dust phenomenon, when compared against models for the formation of rocky terrestrial-like bodies, implies that rocky planet formation in the terrestrial planet zone around Sun-like stars is common.

Rapid disappearance of a warm, dusty circumstellar disk

Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution typically is ignored, although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC 8241 2652 1) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.

A concordance model of the Lyman α forest at z = 1.95

We present 40 fully hydrodynamical numerical simulations of the intergalactic gas that gives rise to the Lyforest. The simulation code, input and output files are available at http://www.cosmos.ucsd.edu/˜gso/index.html. For each simula- tion we predict the observable properties of the H I absorption in QSO spectra. We then find the sets of cosmological and astrophysical parameters that match the QSO spectra. We present our results as scaling relationships between input and output parameters. The input parameters include the main cosmological parameters b, m, �, H0 and �8; and two astrophysical parameters 912 and X228. The parameter 912 controls the rate of ionization of H I, He I and He II and is equivalent to the intensity of the UV background. The second parameter X228 controls the rate of heating from the photoionization of He II and can be related to the shape of the UVB at � < 228 u We show how these input param- eters; especially �8, 912 and X228; effect the output parameters that we measure in simulated spectra. These parameters are the mean flux ¯ F, a measure of the most common Lyline width (b-value) b�, and the 1D power spectrum of the flux on scales from 0.01 - 0.1 s/km. We compare the simulation output to data from Kim et al. (2004) and Tytler et al. (2004) and we give a new measurement of the flux power from HIRES and UVES spectra for the low density IGM alone at z = 1.95. We find that simulations with a wide variety of �8 values, from at least 0.8 - 1.1, can fit the small scale flux power and b-values when we adjust X228 to compensate for the �8 change. We can also use 912 to adjust the H I ionization rate to simultaneously match the mean flux. When we examine only the mean flux, b-values and small scale flux power we can not readily break the strong degeneracy between �8 and X228.

Accretion of a Terrestrial-Like Minor Planet by a White Dwarf

We present optical and infrared characterization of the polluted DAZ white dwarf GALEX J193156.8+011745. Imaging and spectroscopy from the ultraviolet to the thermal infrared indicate that this white dwarf hosts excess infrared emission consistent with the presence of an orbiting dusty debris disk. In addition to the five elements previously identified, our optical echelle spectroscopy reveals chromium and manganese and enables restrictive upper limits on several other elements. Synthesis of all detections and upper limits suggests that GALEX J193156.8+011745 has accreted a differentiated parent body. We compare the inferred bulk elemental composition of the accreted parent body to expectations for the bulk composition of an Earth-like planet stripped of its crust and mantle and find relatively good agreement. At least two processes could be important in shaping the final bulk elemental composition of rocky bodies during the late phases of stellar evolution: irradiation and interaction with the dense stellar wind.

A Triple Protostar System Formed via Fragmentation of a Gravitationally Unstable Disk

Binary and multiple star systems are a frequent outcome of the star formation process and as a result almost half of all stars with masses similar to that of the Sun have at least one companion star. Theoretical studies indicate that there are two main pathways that can operate concurrently to form binary/multiple star systems: large-scale fragmentation of turbulent gas cores and filaments or smaller-scale fragmentation of a massive protostellar disk due to gravitational instability. Observational evidence for turbulent fragmentation on scales of more than 1,000 astronomical units has recently emerged. Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems. The triple protostar system L1448 IRS3B is an ideal system with which to search for evidence of disk fragmentation as it is in an early phase of the star formation process, it is likely to be less than 150,000 years old and all of the protostars in the system are separated by less than 200 astronomical units. Here we report observations of dust and molecular gas emission that reveal a disk with a spiral structure surrounding the three protostars. Two protostars near the centre of the disk are separated by 61 astronomical units and a tertiary protostar is coincident with a spiral arm in the outer disk at a separation of 183 astronomical units. The inferred mass of the central pair of protostellar objects is approximately one solar mass, while the disk surrounding the three protostars has a total mass of around 0.30 solar masses. The tertiary protostar itself has a minimum mass of about 0.085 solar masses. We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 and 320 astronomical units, overlapping with the location of the tertiary protostar. This is consistent with models for a protostellar disk that has recently undergone gravitational instability, spawning one or two companion stars.

TW HYA Association Membership and New WISE-detected Circumstellar Disks

We assess the current membership of the nearby, young TW Hydrae association and examine newly proposed members with the Wide-field Infrared Survey Explorer (WISE) to search for infrared excess indicative of circumstellar disks. Newly proposed members TWA 30A, TWA 30B, TWA 31, and TWA 32 all show excess emission at 12 and 22 μm providing clear evidence for substantial dusty circumstellar disks around these low-mass, ~8 Myr old stars that were previously shown to likely be accreting circumstellar material. TWA 30B shows large amounts of self-extinction, likely due to an edge-on disk geometry. We also confirm previously reported circumstellar disks with WISE and determine a 22 μm excess fraction of 42+10 – 9% based on our results.

THE SOLAR NEIGHBORHOOD. XXVI. AP Col: THE CLOSEST (8.4 pc) PRE-MAIN-SEQUENCE STAR

We present the results of a multi-technique investigation of the M4.5Ve flare star AP Col, which we discover to be the nearest pre-main-sequence star. These include astrometric data from the CTIO 0.9 m, from which we derive a proper motion of 342.0 ± 0.5 mas yr −1 , a trigonometric parallax of 119.21 ± 0.98 mas (8.39 ± 0.07 pc), and photometry and photometric variability at optical wavelengths. We also provide spectroscopic data, including radial velocity (22.4 ± 0.3 km s −1 ), lithium equivalent width (EW) (0.28 ± 0.02 A), Hα EW (−6.0 to −35 A), v sin i (11 ± 1k m s −1 ), and gravity indicators from the Siding Spring 2.3 m WiFeS, Lick 3 m Hamilton echelle, and Keck-I HIRES echelle spectrographs. The combined observations demonstrate that AP Col is the closer of only two known systems within 10 pc of the Sun younger than 100 Myr. Given its space motion and apparent age of 12‐50 Myr, AP Col is likely a member of the recently proposed ∼40 Myr old Argus/IC 2391 Association.