Astrophysics Journal Club


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  Physics and Astronomy
    McMaster University

Current Schedule of Speakers
Fall 2017

Speaker Article Link
September 21
Samantha Benincasa From molecules to young stellar clusters: the star formation cycle across the disk of M 33

We study the association between giant molecular clouds (GMCs) and young stellar cluster candidates (YSCCs) to shed light on the time evolution of local star formation episodes in the nearby galaxy M 33. Methods. The CO (J = 2−1) IRAM all-disk survey was used to identify and classify 566 GMCs with masses between 2 × 104 and 2 × 106 M across the whole star-forming disk of M 33. In the same area, there are 630 YSCCs that we identified using Spitzer-24 µm data. Some YSCCs are embedded star-forming sites, while the majority have GALEX-UV and Hα counterparts with estimated cluster masses and ages. Results. The GMC classes correspond to different cloud evolutionary stages: inactive clouds are 32% of the total and classified clouds with embedded and exposed star formation are 16% and 52% of the total, respectively. Across the regular southern spiral arm, inactive clouds are preferentially located in the inner part of the arm, possibly suggesting a triggering of star formation as the cloud crosses the arm. The spatial correlation between YSCCs and GMCs is extremely strong, with a typical separation of 17 pc. This is less than half the CO (2–1) beam size and illustrates the remarkable physical link between the two populations. GMCs and YSCCs follow the HI filaments, except in the outermost regions, where the survey finds fewer GMCs than YSCCs, which is most likely due to undetected clouds with low CO luminosity. The distribution of the non-embedded YSCC ages peaks around 5 Myr, with only a few being as old as 8–10 Myr. These age estimates together with the number of GMCs in the various evolutionary stages lead us to conclude that 14 Myr is the typical lifetime of a GMC in M 33 prior to cloud dispersal. The inactive and embedded phases are short, lasting about 4 and 2 Myr, respectively. This underlines that embedded YSCCs rapidly break out from the clouds and become partially visible in Hα or UV long before cloud dispersal.
September 28
Matthew Alessi How cores grow by pebble accretion

Planet formation by pebble accretion is an alternative to classical core accretion. In this scenario, planets grow by the accretion of cm-to-m-sized pebbles instead of km-sized planetesimals. One of the main differences with classical core accretion is the increased thermal ablation rate experienced by pebbles. This provides early enrichment to the planet’s envelope, which influences its subsequent evolution and changes the process of core growth. Aims. To describe and compute core growth in the pebble accretion model. We aim to predict core masses and compositions that can form by pebble accretion and compare them to the case of planetesimals. Methods. We have written a code containing both an impact and a planet evolution model to simulate the early growth of a proto-planet self-consistently. The region where high-Z material (in our case SiO2) can exist in vapor form is determined by the temperaturedependent vapor pressure. We include enrichment effects by locally modifying the mean molecular weight of the envelope and determine when direct core growth of the planet terminates. Results. We have identified three phases of core growth in pebble accretion. In the first phase (Mcore < 0.23–0.39 M⊕ ), pebbles impact the core without significant ablation. During the second phase (Mcore < 0.5 M⊕ ), ablation becomes increasingly severe. A layer of high-Z vapor starts to form around the core that absorbs a small fraction of the ablated mass. The rest of the material either rains out to the core or mixes outwards instead, slowing core growth. In the third phase (Mcore > 0.5 M⊕ ), the high-Z inner region expands outwards, absorbing an increasing fraction of the ablated material as vapor. Rainout ends before the core mass reaches 0.6 M⊕ , terminating direct core growth. Conclusions. Our results indicate that pebble accretion can directly form rocky cores up to only 0.6 M⊕ , and is unable to form icy cores. This result contrasts classical core accretion models, which can directly form massive cores of both rocky and icy compositions. Subsequent core growth can proceed indirectly when the planet cools, provided it is able to retain its high-Z material.
October 5
Ryan Chown Mapping the millimeter-wave sky with South Pole Telescope and Planck data

Measurements of fluctuations in the cosmic microwave background (CMB) have revolutionized the field of observational cosmology. Although the CMB dominates the millimeter-wave sky, emission from dusty star-forming galaxies, AGN, dust in our Galaxy, and the thermal Sunyaev-Zel’dovich effect are all significant at millimeter wavelengths. High-sensitivity mm-wave maps from space-based, ground-based, and balloon-borne telescopes have been used to test cosmological models, to study foregrounds, and to study systematics such as CMB lensing. The need for mm-wave maps with high sensitivity over a wide range of angular scales (particularly for lensing analyses) motivated my MSc work, which focused on combining temperature maps from the South Pole Telescope (covering roughly 2500 square degrees) with maps from the Planck Satellite in a nearly optimal way based on anisotropic filtering and noise. I will talk about the components of the mm-wave sky and the combining algorithm. I will show results of the 2500 deg^2 combining analysis, a CMB lensing analysis using one of the combined maps, as well as results of a separate analysis where we combined SPT and Planck data covering the Magellanic Clouds.
October 12
Ian Roberts Research talk
October 19
Samantha Benincasa Practice for UW Astro Lunch Talk & UVic Astro Seminar
October 26
Ashley Bemis TBA
November 2
Fraser The Relationship Between Dust and Star Formation: A Spatially Resolved Perspective in the SAMI Survey
November 9
Collin McNally (tentative)
November 16
Melanie TBA
November 23
Nathan Brunetti TBA
November 30
Ben Pearce TBA
December 7
Kevin Lacaille A SCUBA-2 survey for luminous far-infrared galaxies in protoclusters at z>2