Tuesday, 4th of July

Location: Mason theatre, ground floor

Zoom link: https://macquarie.zoom.us/j/82321436522

Slack channel: #asa2023-galaxies 

Chair: Sarah Brough, Co-Chair: Emily Kerrison

11:00am: Towards the metal free early Universe with the JWST - Themiya Nanayakkara†, Swinburne 

The first JWST/NIRCam observations from JWST has revealed a very blue Universe. The UV slopes are found to be bluer than what traditional stellar population models can predict, leading to possibilities of stellar populations with high nebular continuum escape and/or very low-metallicities. JWST/NIRSpec observations demonstrate the extreme nature of the z>6 emission line galaxies with limited metal content. In my talk I will discuss early results from the JWST GLASS ERS survey and put these results in the context of the local extremely metal poor galaxies observed with VLT/MUSE and Keck/KCWI. 

11:15am: What JWST reveals about the shapes of galaxies at intermediate redshift  - Colin Jacobs†, Swinburne 

Even the very first mid-IR images from JWST have provided a rich opportunity to test our understanding of galaxy evolution. We are now in a position to directly compare this view of the high-redshift universe to studies from previous instruments. In this talk I take a look at insights gained by combining non-parametric morphological measurements with old-fashioned astronomer classifications of galaxies between redshift 1 and 5, and a comparison to HST data that was previously restricted to rest-frame UV below redshift ~2. By selecting for redshift bands where JWST/NIRCam filters align closely with rest-frame optical, we can also compare to a local sample from SDSS. We find that rest-frame UV studies have hitherto masked a large amount of morphological detail, and in compiling an atlas of our sample are also able to test theoretical predictions of the number of disks and bulges in this redshift range. In presenting the atlas I will discuss the surprises that this shift into the IR revealed.

11:30am: New pictures of early massive galaxies from deep JWST infrared imaging and spectroscopy - Karl Glazebrook, Swinburne

The existence of massive (>1E11 Msun stellar mass)  early-type and compact quiescent galaxies at z~4 have been an explosive topic in the last 5 years, as they were originally not predicted by state of the art galaxy formation models. Ground-based spectroscopy with Keck and VLT has only been able to scrape the tip of the bright end of this population, excluding potential older massive objects. I will present new results from JWST  of multiobject spectroscopy with NIRSPEC of a complete sample of 18 objects, with K<24 and 100% redshift confirmation well beyond the ground based limits. In particular we find two exciting new results: a population of very OLD objects with ages of 1 Gyr (forming at 6<z<7) and measurements of very high alpha-enhancement which point to very rapid formation processes in a monolithic starburst. These also provide chemical abundance constraints on early generations of stars. We also constrain the AGN duty cycle, a possible quenching mechanism, in these objects.

11:45am: Gotta Catch ’Em All! – modelling z>~12 star-forming galaxies from JWST - Yuxiang Qin, University of Melbourne

We studied 9 recently discovered JWST galaxies at z>~12 using a semi-analytic model, which was applied to a cosmological simulation that resolves every atomic-cooling galaxy at z<=20 in a volume of 311Mpc side length. We varied model parameters to reproduce the observed UV luminosity function up to z~13, aiming for a statistically representative, modelled, high-redshift galaxy catalogue. Then using the forward-modelled JWST photometry, we identified analogues that resemble the observed ones and studied their properties as well as possible evolutionary paths and local environments.

12:00pm: JWST reveals significant neutral gas absorption in massive z~2 galaxies - Rebecca Davies, Swinburne/ASTRO-3D

Understanding why galaxies stop forming stars is one of the biggest challenges in modern astrophysics. Quenching could be the result of ejective feedback, whereby powerful outflows remove the fuel for star-formation (cold gas), or preventative feedback, whereby outflows and/or jets heat the surrounding gas, preventing it from collapsing to form stars. A key diagnostic of the dominant feedback mode is therefore the cold gas reservoirs of recently quenched galaxies. Most massive galaxies experience quenching at z~2, but the emission from cold dust and molecular gas in high redshift quiescent galaxies is extremely faint, and detailed studies have only been possible for small samples. 

12:15pm: Geostatistics of Galaxies: A novel approach to understand multiscale metallicity variations - Benjamin Metha*, University of Melbourne/ASTRO-3D

The metallicity of a galaxy is set by its gas-phase physics: how is metal-poor gas accreting into a galaxy, how often are stars born, and how well are the products of star-formation mixed into their local environments? With data from high-resolution spectroscopic surveys, we can see that galaxies in the local Universe show significant (~0.1 dex) deviations in metallicity from a simple linear trend, allowing us to directly see local metallicity enrichment and dilution events on scales below 100pc. In this talk, I introduce geostatistics, a family of mathematical tools and techniques that are useful for learning the structure of these small-scale metallicity fluctuations and creating 2D predictive models of a galaxy's metallicity structure from noisy, incomplete data. I will highlight the science opportunities that these models enable, discuss the challenges associated with applying these methods to high-redshift (z~2) galaxies recently observed by JWST, and present a novel forward-modeling approach that may help overcome these challenges.

Poster sparklers in this session:

P27: Sabrina Berger*, University of Melbourne

P58: Bronwyn Reichardt Chu*, Swinburne 

P62: Giulia Cinquegrana*, Monash University

P68: Joachim Krüger*, USQ

P69: Simon Murphy, USQ

Location: T2, second floor

Zoom link: https://macquarie.zoom.us/j/81099211834

Slack channel: #asa2023-stars

Chair: Gayandhi De Silva, Co-Chair: Tanner Wilson

11:00am: Using Asteroseismology to Study Stellar Mass Loss and Multiple Populations in the Globular Cluster M80 - Madeline Howell*, Monash University

Asteroseismology provides a new avenue for accurately measuring the masses of stars, with the detection of their solar-like oscillations. By measuring seismic masses of globular cluster (GC) stars, we can study stellar evolution. We present the first detections of solar-like oscillations in 48 red giants in the GC M80; a metal-poor GC, and only the second with seismic data. Mass loss remains a major uncertainty in stellar modelling. By taking the differences in the averaged masses in various phases of evolution, a total integrated mass loss can be quantified. Mass loss is thought to scale with metallicity, which we test by comparing our M80 results to the seismic masses from a higher-metallicity GC, M4. From this, we find a significant metallicity and temperature dependence in the mass loss on both the RGB and HB. We also investigate if there are mass differences between the multiple populations in M80. We detect a distinct bimodality in the (early) AGB mass distribution which could be a signature of population membership. We report strong evidence for mass loss differences between the populations. Differing mass loss rates on the RGB has been proposed as the second parameter defining the HB morphology variations between GCs. 

11:15am: Solving the puzzle of Fast Yellow Pulsating Supergiants  - May Gade Pedersen, University of Sydney

With its near full-sky photometric survey, the TESS space telescope has opened up exciting new opportunities for studying variable stars all across the HR diagram. This has led to new discoveries such as single sided pulsating stars found in binary systems as well as a new type of pulsating star called Fast Yellow Pulsating Supergiants (FYPS). The FYPS stars provide a valuable opportunity for using asteroseismology to study evolved massive stars, which has previously not been possible. However, the reported pulsation frequencies of the stars are higher than their expected acoustic cutoff frequency, above which oscillation modes are no longer trapped inside the star but dissipate. Therefore, such oscillations should not exist according to theory. With a closer look at the TESS pixel data, I will demonstrate that the reported oscillations are unlikely to be intrinsic to the stars but a result of contamination from nearby variable stars. 

11:30am: How do stellar parameters affect magnetic activity cycles? - Benjamin Montet, University of New South Wales

Observations of stellar magnetic variability similar to the Sun’s 11-year cycle are limited due to the small amplitude of the signal in photometry and the long observing baseline required. While there is a strong theoretical basis for stellar parameters such as mass and metallicity affecting the size, shape, and period of these cycles, observational verification of these predictions are limited. Recently, the high precision and stability of the Kepler and TESS facilities have enabled observations of changing stellar activity levels over a half decade. While this provides relatively modest information for any individual star, their observations of millions of targets enable strong inference across stellar populations. Here, I will present results which suggest a strong relationship between stellar metallicity and the strength of photometric variability, and the implications of this relationship for interpretation of exoplanet transmission spectroscopy observations.

11:45am: Stochastic variables: a lot of stars making a lot of noise - Jeremy Bailey, University of New South Wales

One of the unexpected results from recent high-cadence space photometry missions such as K2, BRITE, and particularly TESS, has been the ubiquity of stochastic variability among luminous stars including blue supergiants, Wolf-Rayet stars and others. These stars vary in a random way with no coherent periods and with a variability spectrum that can be modelled as red noise. They include some of the brightest and best-studied stars in the sky. The cause of variability is the subject of current debate. One theory ascribes it to internal gravity waves in the interior of the star, another to sub-surface convection regions. A further theory is that the variability is due to the clumpy nature of the strong radiatively driven winds from these stars. We present new observations that can help to distinguish between these theories.

12:00pm: Rate Variability of Superflares on G-type Stars - James Crowley*, University of Sydney

Flare occurrence on the Sun is highly variable, exhibiting both short term variations due to the emergence and evolution of active regions, and long term variations from the solar cycle.

Superflares have been observed to occur on solar-like stars, and it is of interest to determine whether solar-like stars exhibit variability in their rates of flaring.

We analyse 283 G-type stars using data from the Transiting Exoplanet Survey Satellite, and identify two stars which exhibit statistically significant changes in the rate of superflare emission. These two stars, TIC 364588501 and TIC 394030788, saw brief outbursts of high flaring activity during which the flaring rate increased by three to four times over the average flaring rate. We discuss reasons for the low number of stars with observed rate variations. 

12:15pm: Machine Learning for Interferometric Imaging - Lucinda Lilley*, University of Sydney

Episodes of dramatic mass loss are known to occur in disparate types of stellar systems, at peak rates of 10^-4 M_solar/year. The duration and extent of mass loss has a profound effect on trajectory of subsequent stellar evolution. A critical observational capability for studying mass loss is imagery at the scale of the wind base – capturing the dynamics here will yield answers to many outstanding questions, such as whether global asymmetries in mass loss shells originate at the stellar surface. Here we present images of stars with dusty circumstellar halos, recovered using the ‘Visible Aperture Masking Polarimetric Interferometer for Resolving Exoplanetary Signatures’ or VAMPIRES instrument, on the SCExAO system of the SUBARU telescope in Hawaii. VAMPIRES recovers interferometric data in polarised light, providing high resolution images of circumstellar dust structures. Here we demonstrate that dense Neural Networks (NN) may be used to predict the physical properties of circumstellar dust structures from the polarised interferometric signals recovered by VAMPIRES, with accuracy and speed that is highly competitive with standard MCMC fitting algorithms. We apply our networks to real VAMPIRES data, to yield new insights into mass loss from several AGB and RSG stars.

Poster sparklers in this session:

P27: Sabrina Berger*, University of Melbourne

P58: Bronwyn Reichardt Chu*, Swinburne 

P62: Giulia Cinquegrana*, Monash University

P68: Joachim Krüger*, USQ

P69: Simon Murphy, USQ

Location: Mason theatre, ground floor

Zoom link: https://macquarie.zoom.us/j/82321436522

Slack channel: #asa2023-galaxies

Chair: Karl Glazebrook, Co-Chair: Pablo Corcho Caballero

2:30pm: Intracluster light - how can we be sure we are measuring what we think we are measuring?  - Sarah Brough, University of New South Wales

Intracluster light (ICL) is the diffuse, extended stellar light observed to spread across the centre of galaxy clusters. Due to the nature of galaxy merging and cluster evolution this diffuse light provides a holistic trace of the evolution of both galaxies and galaxy clusters. However, this light is very faint and challenging to measure and each observer and simulator uses their own measurement method. ICL measurements to date show significant scatter but it is unclear whether this has a physical origin. In this project we are studying what observers and simulators are measuring as ICL, so that when we study this in upcoming mega surveys like the Legacy Survey of Space and Time we can be confident of our analyses. I will present the new understanding revealed by a comparison of the ICL measurements of 8 observers and 3 simulators applied to mock images from 4 cosmological hydrodynamical simulations.

2:45pm: Preparing for low surface brightness studies with LSST: first intra-group light analysis of a GAMA group of galaxies at z=0.2 - Cristina Martinez-Lombilla, Monash University/Swinburne 

The intra-group light (IGL) is the diffuse light that spreads across the center of galaxy groups, concentrated around the brightest central galaxy or BCG. This light is composed of stars unbounded from their host galaxy due to interactions so it forms a fossil record of the dynamical processes a group of galaxies has undergone. Thus, the study of the IGL provides a holistic view of the system's mass assembly history. However, the faint and diffuse nature of the IGL presents a technical challenge as it is typically detected at very low surface brightnesses, hardly detected in past surveys. In this talk, we show a pilot study where we evaluate the potential of Subaru’s Hyper-Suprime Cam (HSC-SSP) data to prepare for low surface brightness (LSB) studies in future large data samples, in particular, from the Rubin Observatory's Legacy Survey of Space and Time (LSST). To do that, we have developed a technique to account for all the systematic effects introduced through the telescope’s optical path and the atmosphere, allowing us to reach a surface brightness limit of 30.8 mag/arcsec^2 in g-band. We will present our results on the first IGL measurement and colour analysis of a group of galaxies at z=0.2. We compare our results with theoretical predictions to infer the dominant formation mechanism of this important LSB component. Finally, we show that with these careful techniques it will be possible to analyse the IGL in an unprecedented sample of groups with LSST.

3:00pm:  The intra-halo light as a probe of galaxy assembly - Katy Proctor*, ICRAR/University of Western Australia

Deep observational studies have revealed that galaxies are surrounded by a diffuse, extended component of stars: the intra-halo light (IHL). The IHL is typically referred to as the stellar halo at the galactic scale, and the intra-group (cluster) light at the scale of galaxy groups (clusters). Regardless of mass scale, the IHL is presumed to be built-up primarily from the disrupted remnants of accreted galaxies and therefore is intrinsically linked to a galaxy’s assembly history. Despite recent observational advances, the low surface brightness nature of the IHL hampers efforts to obtain a statistically representative sample from extra-galactic systems, making it difficult to interpret observations in a broader cosmological context. We address this issue using a large sample of galaxies drawn from the EAGLE simulation. We employ a novel galaxy decomposition technique based on stellar kinematics and binding energy that we have specifically optimised to detect the IHL component of galaxies covering a broad range in halo mass and environment. In this talk, I will discuss how well the IHL component can be separated from the central galaxy, and how the properties of this component varies with host halo mass, ultimately addressing the question: how is the present day IHL connected to a galaxy’s assembly history?

3:15pm: Tracing the baryons in galaxy groups - Ajay Dev*, ICRAR/University of Western Australia

The majority of the baryons in the Universe lie outside of galaxies. Hence to get towards an accurate understanding of the baryon density, we need to start looking at the broader environment of haloes, filaments and eventually voids. In this presentation, I will describe the first results in which we measure the baryon content (hot gas, stars and neutral gas) as a function of halo mass. Our approach involves the construction of scaling relations between the baryon component mass (e.g., plasma, stars or HI) versus halo mass. After establishing the scaling relations, we convolve these relations with the recently published GAMA+SDSS+REFLEX II Halo Mass Function, to provide a breakdown of the baryon budget as a function of halo mass. Our results show that the high mass haloes are dominated by plasma, then stars, then neutral gas (as expected), while this ratio looks to transition to HI, stars and plasma as we move to lower mass haloes. This highlights how the halo environment is remarkably different across halo masses and that this is likely to be a key factor in understanding galaxy formation and evolution. We conclude by showing the latest

baryon budgets derived from our halo-centric perspective, and from which we infer how the hard to observe warm plasma mass component varies with halo mass. We will also discuss plans to extend this work by (a) the stacking of eROSITA and ASKAP data, and (b) by extending the GAMA group catalogue via the upcoming 4MOST WAVES survey.

Poster sparklers in this session:

P22: Kamal Bora*, Swinburne University

P40: Angel Chandro Gomez*, ICRAR 

P61: Meghna Mukesh Menon*, Macquarie University

Location: T2, second floor

Zoom link: https://macquarie.zoom.us/j/81099211834

Slack channel: #asa2023-stars

Chair: Ben Montet, Co-Chair: Jackie Bondell

2:30pm: Stellar spots cause measurable variations in atmospheric metallicity - Tanner Wilson*, Monash University

The detailed chemical abundances of stellar populations can be used to better understand nucleosynthesis, galactic chemical enrichment, and stellar evolution. High-quality spectra and an accurate stellar atmosphere model are required to precisely measure a star's atmospheric parameters and chemical abundances. Stellar spots will introduce variations to the spectra of stars that are not universally accounted for when performing spectroscopic inference. In this talk, I discuss the effect of stellar spots on the precise inference of metallicity.

We have found that fitting spotted spectra with a non-spotted model introduces a scatter of up to 0.05 dex in the inferred metallicity, particularly for stars with high fractional surface spot coverage. This bias is comparable to other effects, including atomic diffusion and the absence of local thermodynamic equilibrium. Stellar spots can introduce a systematic uncertainty in metallicity that is not presently included in spectroscopic analysis, which could limit or bias inference in population-level studies and differential abundance analyses.

2:45pm: Are solar flares the result of stress accumulating until a threshold is reached? - Julian Carlin*, University of Melbourne

Solar flares involve a sudden release of energy from the magnetic field of the Sun's corona. A classic prediction is that the waiting time until the next flare should be correlated with the size of the previous flare, if the stress in the system approaches a threshold before each flare is triggered. We find this cross-correlation is broadly absent from flares recorded by the Geostationary Operational Environmental Satellite, looking back over 50 years. Interpreting flare event statistics through the lens of a state-dependent Poisson process, in which the instantaneous flare rate is a function of the stress of the system, we propose a series of heuristic tests to search for signatures of a stress-accumulation and relax process. Our results imply at least one of the following: i) the threshold at which a flare is triggered varies in time; ii) the rate at which energy is driven into active regions varies in time, iii) flare catalogs are incomplete; or iv) the description of solar flares as resulting from a build-up and release of energy, once a threshold is reached, is incomplete.

3:00pm: Using astrochemistry to understand orbital properties of AGB stars - Taïssa Danilovich, Monash University

Asymptotic giant branch (AGB) stars are a late evolutionary stage of sun-like stars. Studies resolving the circumstellar envelopes of AGB stars have revealed spirals, discs and bipolar outflows, with the shaping of the gas attributed to interactions with a companion. Previous studies have focused on density structures in the wind to try to understand companion parameters. I will show a new method using a combined chemical and dynamical analysis which we used to reveal a highly eccentric and long-period (thousand-year) orbit for W Aquilae, a binary system containing an AGB star and a main sequence companion. Our results are based on features and molecules that formed during periastron interactions. This new method can yield stringent constraints on the orbital parameters of long-period binaries containing AGB stars, and establishes a template for future studies. 

3:15pm: Common envelopes in massive stars: fundamental advances and implications - Ilya Mandel, Monash University

Dynamically unstable mass transfer episodes, also known as common envelope phases, play a key role in the evolution of massive stellar binaries, including X-ray binaries, binary radio pulsars, and gravitational-wave sources. In this talk, I will introduce insights into common-envelope evolution motivated by detailed 3-d hyrodynamics models and a new framework for translating these insights into practical recipes for binary evolution that can be applied to rapid binary population synthesis. I will then discuss several implications of this new framework, which can help us to understand phenomena as diverse as kicks of low-mass X-ray binaries and the chirp mass distribution of merging binary black holes.

Poster sparklers in this session:

P22: Kamal Bora*, Swinburne University

P40: Angel Chandro Gomez*, ICRAR 

P61: Meghna Mukesh Menon*, Macquarie University

Location: Mason theatre, ground floor

Zoom link: https://macquarie.zoom.us/j/82321436522

Slack channel: #asa2023-galaxies

Chair: Scott Croom, Co-Chair: Katy Proctor

4:00pm: MAUVE-over M82, there is new outflow on the street - Adam Watts†, ICRAR/University of Western Australia

It is well established that galactic outflows play a key role in regulating star formation, and in chemically enriching galaxies and their CGM. This ability for pc-scale processes to affect the kpc-scale properties of galaxies means that we must resolve outflows at a high spatial resolution to understand how efficiently they eject metal-enriched gas out of galaxies. However, our understanding of outflows on 10s-100s of pc is dominated by just two nearby galaxies where the outflow can be sufficiently resolved, M82 and NGC253. I will present the first results from the new VLT/MUSE survey MAUVE(MUSE and ALMA Unveiling the Virgo Environment), where we report the discovery of a massive, starburst-driven outflow extending almost 5kpc from the nearby galaxy NGC4383. Hosting the largest gas disc in the Virgo cluster, our discovery makes NGC4383 the only known example of a gas-rich intermediate-mass galaxy in the local Universe hosting such an outflow, making it a powerful target to study feedback physics. With the high spatial resolution of MUSE, I will characterise the properties of the ionised gas outflow on ~100pc scales, which shows an increasingly complex kinematic structure above and below the galaxy disc. Deriving the gas-phase metallicity in the same physical regions, we find that the outflow does not contain heavily metal-enriched gas compared to the central disc. I will conclude by discussing how these results fit into our understanding of the physics of stellar feedback.

4:15pm: Hunting for young obscured AGN with Bayesian SED fitting - Emily Kerrison*, University of Sydney/CSIRO

The spectral energy distribution (SED) of an AGN reveals important details about its multi-phase gaseous environment and its emission mechanisms. By focusing on multiwavelength emissions in the radio regime, which trace the cool and warm circumnuclear medium, coupled with observations of the hot gas revealed at X-ray wavelengths, SED analysis can advance our understanding of both AGN physics and composition. This approach is particularly useful for understanding the most compact sources, which cannot be spatially resolved to reveal their morphological details. I present here a Bayesian modelling framework specifically tailored to broadband radio SEDs, and which I am applying to large, homogeneous samples of radio AGN with a view to eventually identifying a complete, southern-sky sample of these objects. These samples will form the basis of further studies into gaseous emission and absorption mechanisms at both radio and X-ray wavelengths as part of the Studies of eROSITA and ASKAP-FLASH Obscured Galaxies project (SEAFOG). The wide-field coverage of both eROSITA and FLASH, combined with the uniform construction of these broadband SEDs, mean this work overcomes many of the challenges faced in analysing small, inhomogeneous samples, which have historically been limited by a lack of both sky and frequency coverage. The scalability of this framework makes it an exciting tool for multiwavelength astronomers as both ASKAP and eROSITA move forward with all-sky surveys.

4:30pm: Cold neutral gas in the surroundings of AGNs - Aditya JNHS, University of Sydney

In this talk, I will report new detections of AGN associated HI 21cm absorption at 0.4 <z <1.0 from pilot observations of ASKAP First Large Absorption Survey for HI (FLASH), discuss the properties of these absorbers, and outline the plans and predictions for the full survey that began earlier this year. In the pilot surveys, we detected the strongest known absorber reported till date, associated with a compact radio source. HI 21cm absorption is an effective technique to probe the distribution and kinematics of neutral gas in high-redshift AGN host galaxies. Studies so far have found a significant drop in the detection rate and strength of HI 21cm absorption between z<0.4 and z>1.0 radio sources, the primary reasons for which are still not clear.

FLASH survey covers the entire southern sky, and probes systems at 0.4<z<1.0 where the `transition' in the detection rate is occurring. The survey will be critical for understanding the effects of redshift evolution and AGN luminosity on neutral gas properties. In addition, I will present recent eMERLIN observations towards a local galaxy merger, where we trace in-falling neutral hydrogen gas at the radio core, and make a case for cold hydrogen fuelling a second episode of jet activity.

4:45pm:  Luminous High-redshift Quasars: their Black Holes and Accretion Discs - Samuel Lai*, Australian National University

Luminous Active Galactic Nuclei (AGN) are crucial in the study of black hole (BH) growth and host galaxy co-evolution in the high-redshift universe. With multiwavelength rest-frame optical/UV data from 100+ quasars between 3.5 < z < 7.5, we discuss the information encoded in velocity-broadened spectral features traced back to ionised gas in close proximity to the BH accretion disk, which allows us to estimate BH masses, accretion rates, outflow signatures, and chemical abundances in the accretion discs of some of the most luminous objects in the universe. Furthermore, by utilising ray-traced models of the multi-temperature thermal accretion disc emission around Kerr BHs, we present a complementary method to constrain BH properties using Bayesian inference. We demonstrate how these two independent methods reliably reproduce BH masses from reverberation mapping experiments and highlight how the latter method is significantly less costly. We release a spectral modelling software package and the black hole accretion disc modelling code for public use, demonstrating their application not just for AGN, but also in emission and absorption-line studies, such as diffuse gas metallicity in the interstellar medium of nearby galaxies.

5:00pm: Gamma-Ray Narrow Line Seyferts are Strange - XShooter data on PKS2004-447 recent flare - Wei Jeat Hon*, University of Melbourne

Narrow Line Seyferts (NLS1) are early-stage AGN, of which 7% are known to possess relativistic jets and are identified as gamma-ray sources. Despite the importance of understanding how the jet can interact with the BLR, this aspect has not yet been thoroughly investigated in most jetted AGN. In my talk, I will introduce the unique case of PKS 2004-447, the first observation of such an interaction. During a recent high-energy flare from the jet, a XShooter observation captured a flux excess in the broad lines with a redshift of up to 250 km/s. This 'red-excess' was no longer visible 1.5 years later. This discovery suggests that the relativistic jet can affect the physics of the BLR in this peculiar AGN, and that gamma-ray production can lead to the formation of additional and localized broad emission components. Our results highlight the importance of optical spectroscopy for flaring jetted AGN and provide a starting point for future dedicated studies of this kind.

5:15pm: Ageing and Quenching in the Local Universe - Pablo Corcho-Caballero*, Macquarie University

The bimodal distribution of galaxies with respect to various physical quantities, such as star formation rate (SFR), is often attributed to "quenching" processes that halt star formation. However, distinguishing between galaxies that undergo sudden quenching and those that experience gradual secular evolution (ageing) is a daunting task. Our recent series of papers (Corcho-Caballero+21b, 23a, 23b) presents an innovative observational approach that can differentiate between these two types of evolution and explore their correlation with other physical properties. Using the Ageing Diagram, which combines various star formation proxies that are sensitive to different timescales, we can examine the population of galaxies that have recently ceased forming stars and compare it with theoretical models (e.g. IllustrisTNG) to obtain a more comprehensive understanding of the star formation history of galaxies. In this talk, I will present the key findings of this study, showcasing the diverse galaxy populations that are affected by quenching processes and their underlying physical mechanisms.

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

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