Location: T2, second floor
Zoom link: https://macquarie.zoom.us/j/81099211834
Slack channel: #asa2023-stars
Chair: May Gade Pederson, Co-Chair: Jaime Alvarado Montes
4:00pm: The magnetic evolution of solar-type stars - Emma Brown*†, University of Southern Queensland/Central Queensland University
The dynamo-driven magnetic fields of the Sun and other solar-type stars power a range of activity phenomena, including dark surface spots and the magnetised stellar winds that impact on circumstellar environments. It is well established that the magnetic fields of Sun-like stars weaken over billions of years, and that today’s Sun is much less active compared to in its early history. What has not been clear is whether the young Sun had smooth magnetic cycles like it does today, how solar-type stars evolve from active to inactive, and whether this transition is smooth or marked by distinct phases. This talk will give an overview of the latest observational constraints on the dynamos operating in both young and mature main sequence solar-type stars. This includes the first observations of solar-like magnetic fluctuations for a young and extremely active solar analogue, and new evidence for a distinct change in dynamo properties that occurs around the middle of the main sequence for G stars. These results provide valuable insight into the potential history and evolution of our Sun’s magnetic dynamo, and the potential conditions within our space environment.
4:15pm: Identifying Heliospheric Structures: Interplanetary Scintillation with the Murchison Widefield Array - Angie Waszewski*†, ICRAR-Curtin/CSIRO
Interplanetary scintillation (IPS) is a phenomenon that causes small radio sources to “twinkle” in the solar wind, making it a useful tool for studying space weather. Recently, we have revitalised this old technique by adapting it for modern low-frequency instruments like the Murchison Widefield Array (MWA). Our key advancement is utilising the MWA's large field of view, enabling us to monitor all IPS sources within a 30-degree field, resulting in an unprecedented density of measurements, around 250 sources per 900 sq deg. This has led to the launch of the IPS space weather era of the MWA, with the first detection and characterization of a coronal mass ejection.
In addition, a recent survey of IPS sources above the MWA has provided the basis for our work. We conducted a blind search of 49 days of MWA IPS survey observations from mid-2019, revealing interesting structures characterised by higher than usual scintillation levels, despite being taken during solar minimum. One solar wind enhancement was observed in two observations several hours apart, allowing us to infer the plane-of-sky velocity.
After careful examination of solar event catalogues and white-light coronagraph images, we determined that this observed heliospheric structure is not caused by a coronal mass ejection, but is most likely a co-rotating interaction region. This demonstrates the potential of the MWA in studying the dynamics of the heliosphere and improving our understanding of space weather phenomena.
4:30pm: Detection of radio emission from stars via proper-motion searches - Laura Driessen, University of Sydney
I present a method for identifying radio emission from stars using their proper-motion and wide-field radio surveys. Stellar radio emission can reveal information about their coronae, magnetic fields, binary interactions, and possibly star-planet interactions. Identifying radio stars is challenging because the probability of chance coincidence between an optical star and a background radio galaxy is high. It is therefore important to develop new methods for definitively identifying radio emission from stars. This is the first time proper-motion has been used in wide-field radio surveys to identify radio stars. We used Gaia astrometry to identify stars that have proper motions. We then used the time elapsed between FIRST and RACS-mid, and FIRST and VLASS to identify radio sources that have moved across the sky. We demonstrated the efficacy of this method by finding 8 radio stars, 2 of which had not previously been identified. We investigated the time-baselines required to detect the eight sources we found using current and future instruments, such as the MeerKAT and the SKA. I will present this proper-motion method for finding radio stars, the results of the search and the future prospects for this search method.
4:45pm: Detection of radio emission from ultracool dwarfs via circular polarisation searches - Kovi Rose*, University of Sydney
I describe a method for identifying radio emission from ultracool dwarfs (UCDs) in all sky MHz to GHz surveys and present the coolest dwarf detected in radio.
Stars and pulsars are the only known sources of significant circularly polarised emission. Coherent emission from UCDs tends to be highly circularly polarised and is tied to UCD magnetospheric dynamics. This emission is often rotationally modulated and can be used to measure UCD rotational periods. Auroral currents driving UCD emission are generated by co-rotational breakdown between the magnetosphere and circumstellar plasma, or as the result of magnetospheric interaction with a companion. Radio studies of UCDs can thus be used to probe magnetic fields and to identify binaries or exoplanets.
We used the ASKAP RACS-mid survey to identify circularly polarised sources. We found 32 pulsars and 65 stars; 51 of which had not been previously detected in radio. We conducted ATCA observations of an unknown source later identified as a T8 dwarf. We measured periodic, rotationally modulated coherent emission from this object – the coolest dwarf detected in radio.
Our method can be used with future all sky surveys to detect and study UCDs and their companions.
5:00pm: Classical Novae in the ASKAP Pilot Surveys - Ashna Gulati, University of Sydney
In a semi-detached binary system, Roche lobe overflow from a late type main sequence star accumulates on a White Dwarf, igniting a thermonuclear fusion reaction that results in a roughly spherical shell ejecta outflow. These eruption events, called classical novae, play a significant role in the Galaxy’s chemical enrichment, facilitate star formation and serve as nearby probes for several astrophysical processes such as binary mass transfer and explosive nuclear burning. The receding radio photosphere through the expanding ejecta samples the whole ejecta mass profile by interacting with it making radio observations ideal tracers of the expanding ejecta.
In this talk, I will present results from a systematic search for radio counterparts of novae using the Australian Square Kilometre Array Pathfinder (ASKAP). I will demonstrate how the sensitivity of ASKAP surveys, as well as their surveying nature over multiple epochs and over ~5000 square degrees on the sky, can contribute to unbiased radio observations over most novae-concentrated regions especially those with high optical extinction, further eliminating the need for large-scale radio campaigns that span decades. These unbiased observations may help remove uncertainty in predicting accurate ejecta masses and serve as the true test of nova theory, as well as help place strict limits on the fraction of novae that generate synchrotron radiation, as captured in follow-up of two ASKAP detected novae in our sample.
5:15pm: What happens to accretion disks when they are engulfed into a stellar envelope? - Ana Lourdes Juarez Garcia*, Macquarie University
One of the most important phases during the evolution of a close binary system is the common envelope (CE), which is defined as a binary interaction whereby a compact companion inspirals into the envelope of a large giant star. Before the onset of the CE phase, accretion from the primary onto the companion star forms an accretion disk. Once the system enters the CE phase, material from the primary will interact with the accretion disk. This interaction has not been studied in detail, and it is unclear if the disk will survive or will evaporate. The presence of a disk may have an impact on the formation of jets before and during the CE phase, this can alter the dynamics during the inspiral process and the outcome of the CE interaction, such that it is needed to be included in simulations in order to predict the outcome of CE interactions correctly. In this work, we carried out 3D hydrodynamic numerical simulations of an accretion disk around a star in a binary system formed by a main-sequence star (7 M_sun) and a neutron star (1.4 M_sun), I will discuss the results of the numerical simulations and the phenomena that the pre-existing disk produce in the CE.
Poster sparklers in this session:
P24: Alma Maria Sebastian*, Swinburne
P57: Henry Zovaro, ANU
P65: Alexander Wallace, Monash University
P66: Deepak Chahal, Macquarie Uni.
P96: Anuj Gautam, USQ