arXiv subjects

Julia Martikainen

Publications and source records attributed to Julia Martikainen.

5 recordsLinked to original sources

arXiv2026

Exploring Polarized Millimeter Emission from Protoplanetary Disks with Irregular Dust Grains

Polarization at millimeter wavelengths provides a powerful diagnostic of dust grain properties in protoplanetary disks. Standard models based on solid spherical grains often struggle to reproduce the observed polarization fractions and morphologies in systems where self-scattering is expected to dominate. We investigate the impact of grain morphology on polarized millimeter emission by comparing models that adopt solid spherical grains with models that employ solid irregular hexahedral particles drawn from the TAMUdust2020 database. Both grain populations share identical size distributions, enabling us to isolate the effects of geometry while preserving the same internal structure and material density. We explore three optical-depth regimes-optically thick, optically thin, and an intermediate hybrid case-to assess how grain morphology modifies the polarization structure under different conditions. For size distributions with $a_{\mathrm{max}} \sim λ/ 2π$, where scattering-induced polarization is expected to peak, we find that the polarization morphology and fraction are nearly indistinguishable between spherical and irregular grains. The primary quantitative difference is an enhancement of the scattering opacity by up to a factor of $\sim 2.5$ for irregular particles, implying that disk dust masses inferred under the assumption of spherical grains may be systematically overestimated. Irregular grains also suppress the polarization reversal predicted by Mie theory at large size parameters ($x>1$). Nevertheless, modifying grain geometry alone is insufficient to reproduce the observed polarization fractions within a pure self-scattering framework. These results suggest that additional physical effects, such as dust porosity, warrant dedicated investigation.

astro-ph.SR↗

arXiv2026

Classifying white dwarfs from multi-object spectroscopy surveys with machine learning

With tens to hundreds of spectra of white dwarfs being taken each night from multi-object spectroscopic surveys, automated spectral classification is essential as part of efficient data processing. In this study, we design a neural network to classify the spectral type of white dwarfs using a combination of spectra from the Dark Energy Spectroscopic Instrument (DESI) data release~1 and imaging from Pan-STARRS photometry. The trained network has a near 100% accuracy at identifying DA and DB white dwarf spectral types, while having an 85-95% accuracy for identifying all other primary types, including metal pollution. Distinct spectral or photometric features map into separate structures when performing a Uniform Manifold Approximation and Projection (UMAP) dimensionality reduction. Investigating further and looking at multiple epoch spectra, we performed a separate search for objects that have strongly changing spectral signatures using UMAP, discovering 3 new inhomogeneous surface composition ('double-faced') white dwarfs in the process. We lastly show how machine learning has the potential to separate single white dwarfs from double white dwarf binary star systems in a large dataset, ideal for isolating a single star population. The results from all of these techniques show a compelling use of machine learning to boost efficiency in analysing white dwarfs observed in multi-object spectroscopy surveys, at times replacing the need for human-driven spectral classifications. This demonstrates our techniques as powerful tools for batch population analyses, finding outliers as a form of rare subclass detection, and in conducting multi-epoch spectral analyses.

astro-ph.SR↗

arXiv2024

Optical and Radio Analysis of Systematically Classified Broad-lined Type Ic Supernovae from the Zwicky Transient Facility

We study a magnitude-limited sample of 36 Broad-lined Type Ic Supernovae (SNe Ic-BL) from the Zwicky Transient Facility Bright Transient Survey (detected between March 2018 and August 2021), which is the largest systematic study of SNe Ic-BL done in literature thus far. We present the light curves (LCs) for each of the SNe, and analyze the shape of the LCs to derive empirical parameters, along with the explosion epochs for every event. The sample has an average absolute peak magnitude in the r band of $M_r^{max}$ = -18.51 $\pm$ 0.15 mag. Using spectra obtained around peak light, we compute expansion velocities from the Fe II 5169 Angstrom line for each event with high enough signal-to-noise ratio spectra, and find an average value of $v_{ph}$ = 16,100 $\pm$ 1,100 km $s^{-1}$. We also compute bolometric LCs, study the blackbody temperature and radii evolution over time, and derive the explosion properties of the SNe. The explosion properties of the sample have average values of $M_{Ni}$ = $0.37_{-0.06}^{+0.08}$ solar masses, $M_{ej}$ = $2.45_{-0.41}^{+0.47}$ solar masses, and $E_K$= $4.02_{-1.00}^{+1.37} \times 10^{51}$ erg. Thirteen events have radio observations from the Very Large Array, with 8 detections and 5 non-detections. We find that the populations that have radio detections and radio non-detections are indistinct from one another with respect to their optically-inferred explosion properties, and there are no statistically significant correlations present between the events' radio luminosities and optically-inferred explosion properties. This provides evidence that the explosion properties derived from optical data alone cannot give inferences about the radio properties of SNe Ic-BL, and likely their relativistic jet formation mechanisms.

astro-ph.HE↗

arXiv2023

Ejected Particles after Impact Splash on Mars: Aggregates and Aerodynamics

Our earlier laboratory measurements showed that low-velocity sand impacts release fine <5 μm dust from a Martian simulant soil. This dust will become airborne in the Martian atmosphere. Here, we extend this study by measuring aerodynamic properties of ejecta and characterizing deviations from the behavior of spherical, monolithic grains. We observe the settling of particles emitted as part of an impact splash. The sizes (20 to 280 μm) and sedimentation velocities (0.1 to 0.8 ms^{-1} ) of the particles are deduced from high-speed videos while the particles sediment under low ambient pressure of about 1 mbar. The particles regularly settle slower than expected, down to a factor of about 0.3. Using optical microscopy, the shape of the captured particles is characterized by simple axis ratios (longest/smallest), which show that the vast majority of particles are irregular but typically not too elongated, with axis ratios below 2 on average. Electron microscopy further reveals that the particles are typically porous aggregates, which is the most likely reason for the reduction of the sedimentation velocity. Due to the reduced bulk density, aggregates up to 10 μm in diameter should regularly be a part of the dust in the Martian atmosphere.

astro-ph.EP↗

arXiv2022

Releasing Atmospheric Martian Dust in Sand Grain Impacts

Emission of dust up to a few micrometer in size by impacts of sand grains during saltation is thought to be one source of dust within the Martian atmosphere. To study this dust fraction, we carried out laboratory impact experiments. Small numbers of particles of about 200\textmu{}m in diameter impacted a simulated Martian soil (bimodal \textit{Mars Global Simulant}). Impacts occurred at angles of $\sim 18^\circ$ in vacuum with an impact speed of $\sim 1 \rm m/s$. Ejected dust was captured on adjacent microscope slides and the emitted particle size distribution (PSD) was found to be related to the soil PSD. We find that the ejection of clay sized dust gets increasingly harder the smaller these grains are. However, in spite of strong cohesive forces, individual impacts emit dust of 1\textmu{}m and less, i.e. dust in the size range that can be suspended in the Martian atmosphere. More generally, the probability of ejecting dust of a given size can be characterized by a power law in the size range between 0.5\textmu{}m and 5\textmu{}m (diameter).

astro-ph.EP↗