Methanol Maser Emission from Galactic Center Sources with Excess 4.5 {\mu}m Emission

We present a study of signatures of on-going star formation in a sample of protostellar objects with enhanced 4.5 {\mu}m emission ('green' sources) near the Galactic center. To understand how star formation in the Galactic center region compares to that of the Galactic disk, we used the Expanded Very Large Array to observe radiatively excited Class II 6.7 GHz CH3OH masers and collisionally excited Class I 44 GHz CH3OH masers, both tracers of high-mass star formation, toward a sample of 34 Galactic center and foreground 'green' sources. We find that 33\pm15% of Galactic center sources are coincident with 6.7 GHz masers, and that 44\pm17% of foreground sources are coincident with 6.7 GHz masers. For 44 GHz masers, we find correlation rates of 27\pm13% and 25\pm13% for Galactic center green sources and foreground green sources, espectively. Based on these CH3OH maser detection rates, as well as correlations of green sources with other tracers of star formation, such as 24 {\mu}m emission and infrared dark clouds (IRDCs), we find no significant difference between the green sources in the Galactic center and those foreground to it. This suggests that once the star formation process has begun, the environmental differences between the Galactic center region and the Galactic disk have little effect on its observational signatures. We do find, however, some evidence that may support a recent episode of star formation in the Galactic center region.Comment: 73 pages, 34 figures, 5 tables. Accepted for publication in Ap

The enigmatic core L1451-mm: a first hydrostatic core? or a hidden VeLLO?

We present the detection of a dust continuum source at 3-mm (CARMA) and 1.3-mm (SMA), and 12CO(2-1) emission (SMA) towards the L1451-mm dense core. These detections suggest a compact object and an outflow where no point source at mid-infrared wavelengths is detected using Spitzer. An upper limit for the dense core bolometric luminosity of 0.05 Lsun is obtained. By modeling the broadband SED and the continuum interferometric visibilities simultaneously, we confirm that a central source of heating is needed to explain the observations. This modeling also shows that the data can be well fitted by a dense core with a YSO and disk, or by a dense core with a central First Hydrostatic Core (FHSC). Unfortunately, we are not able to decide between these two models, which produce similar fits. We also detect 12CO(2-1) emission with red- and blue-shifted emission suggesting the presence of a slow and poorly collimated outflow, in opposition to what is usually found towards young stellar objects but in agreement with prediction from simulations of a FHSC. This presents the best candidate, so far, for a FHSC, an object that has been identified in simulations of collapsing dense cores. Whatever the true nature of the central object in L1451-mm, this core presents an excellent laboratory to study the earliest phases of low-mass star formation.Comment: 15 pages, 9 figures, emulateapj. Accepted by Ap

Forward displacements of fading objects in motion: the role of transient signals in perceiving position

Visual motion causes mislocalisation phenomena in a variety of experimental paradigms. For many displays objects are perceived as displaced 'forward' in the direction of motion. However, in some cases involving the abrupt stopping or reversal of motion the forward displacements are not observed. We propose that the transient neural signals at the offset of a moving object play a crucial role in accurate localisation. In the present study, we eliminated the transient signals at motion offset by gradually reducing the luminance of the moving object. Our results show that the 'disappearance threshold' for a moving object is lower than the detection threshold for the same object without a motion history. In units of time this manipulation led to a forward displacement of the disappearance point by 175ms. We propose an explanation of our results in terms of two processes: Forward displacements are caused by internal models predicting positions of moving objects. The usually observed correct localisation of stopping positions, however, is based on transient inputs that retroactively attenuate errors that internal models might otherwise cause. Both processes are geared to reducing localisation errors for moving objects

Evolutionary Signatures in the Formation of Low-Mass Protostars. II. Towards Reconciling Models and Observations

A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars, using the Young & Evans (2005) model as our starting point. We calculate the radiative transfer of the collapsing cores throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions. We find that scattering, 2-D geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. A cycle of episodic mass accretion, however, can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function (CMF) and stellar initial mass function (IMF). The combination of outflow cavities and episodic mass accretion reduce the connection between observational Class and physical Stage to the point where neither of the two common observational signatures (bolometric temperature and ratio of bolometric to submillimeter luminosity) can be considered reliable indicators of physical Stage.Comment: 27 pages. Accepted for publication in Ap

The partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism, Glen Roy, Scotland

Garnet porphyroblasts in sillimanite-bearing pelitic schists contain complex textural and compositional zoning, with considerable variation both within and between adjacent samples. The sillimanite-bearing schists locally occur in regional Barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism. Garnet in these Dalradian rocks from the Scottish Highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution- reprecipitation followed by partial dissolution. Individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co-exists with those containing largely unmodified characteristic bell-shaped Mn-profiles. This highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation. Cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite-producing reactions. The modification of garnet via a dissolution-reprecipitation process releases Ca into the effective whole rock composition, displacing the pressure-temperature positions of subsequent isograd reactions. This represents the first report of internal metasomatism controlling reaction pathways. The behaviour of garnet highlights the importance of kinetic factors, especially deformation and fluids, in controlling reaction progress and how the resulting variability influences subsequent prograde history. The lack of a consistent metamorphic response, within and between adjacent schists, suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions