Supernovae (SNe) enrich galaxies with metals, have influence on births of new stars and evolution of low mass galaxies that is mainly driven by feedback from SNe explosions. Therefore they are crucial for the development of the universe. My current research is related to understanding the progenitor systems of SNe Ia, which we use as standard candles to measure the expansion of the universe, and Superluminous Supernovae (SLSNe), which are likely connected to deaths of supermassive metal poor stars in pristine environments.
Spectropolarimetry of Supernovae Ia
Understanding the effect of dust extinction on SNe Ia magnitudes is essential for accurate measurement of cosmological parameters and the expansion history of the Universe. Studying dust along sight lines to SNe Ia might also lead to conclusions on the progenitor systems, because different progenitor systems imply different circumstellar environments.
Intriguingly, studies of the host-galaxy dust extinction from SNe Ia yielded diverse values of the total-to-selective extinction ratio, often $R_V \lesssim 2$, significantly lower than the average $R_V$ of 3.1 for Milky Way dust. We have come to the same result using a statistical approach, by developing color excess probabilities $E(B-V)$ as a function of galactocentric distance for different galaxy morphology groups, and applying them to a sample of SN Ia $E(B-V)$ observations to determine the average $R_V$ value (Cikota+ 2016).
Furthermore, spectropolarimetry is a powerful technique that enables us to study interstellar and circumstellar dust along sight lines to SNe Ia (by measuring the continuum polarization), and the supernova Ia ejecta asymmetry (by measuring polarization of absorption lines). The wavelength of the continuum polarization peak, $\lambda_{max}$, depends on the dust grain size distribution (Serkowski+ 1975).
Figure 1: Both, SNe Ia and PPNe, have steeply rising polarization curves towards blue wavelengths with $\lambda_{max} \lesssim 0.4 \mu m$, compared to HD 141318, a normal Galactic star with $\lambda_{max} \sim$ 0.55 $\mu m$. Source: Cikota+ 2017c.
Figure 1 shows that highly reddened SNe Ia with low $R_V$ values, display peculiar continuum polarization wavelength dependencies, steeply rising towards the blue, with polarization peaks at short wavelengths ($\lambda_{max} \lesssim 0.4 \mu m$, Patat+ 2015), which in general implies an enhanced abundance of small dust grains. For comparison, normal sight lines to Milky Way stars have $\lambda_{max} \sim 0.55 \mu m$. It is not well understood why SNe Ia sight lines display such different polarization profiles compared to what we observe in the Milky Way. Possible explanations are that the composition of interstellar dust in SNe Ia host galaxies is different from the dust in our Galaxy, or that there is circumstellar dust with an enhanced abundance of small grains, which was ejected from the progenitor system before the explosion. Scattering from CSM dust is also a possibility.
We undertook spectropolarimetry of Galactic stars with anomalous extinction sightlines and low $R_V$ values, in order to possibly find a similarity to SNe Ia. However, we found that they have normal polarization curves (Cikota+, submitted).
On the other hand, we noticed that some post-AGB stars (proto-planetary nebulae, PPNe), which also may play an important vague role on the evolutionary path of some SNe Ia (Jones & Boffin 2017), have remarkably similar polarization curves to those observed towards highly reddened SNe Ia (Figure 1). These polarization curves in PPNe are produced by CSM scattering (Oppenheimer+ 2005). Thus, we suggest that also some SNe Ia polarization curves might be produced by CSM dust scattering (Cikota+ 2017c). Furthermore, we speculate that those SNe Ia might have exploded within a PPN, and be observational evidence for the core-degenerate progenitor scenario (Kashi & Soker 2011), in which a white dwarf merges with the core of a companion AGB star.
Currently, I am working on a statistical analysis of 30 SNe Ia, observed at over 190 epochs with VLT/FORS. We are studying the ejecta asymmetry by looking at absorption line polarization, and aim to compare our observations to polarization simulations from Mattia Bulla (Univ. of Stockholm), in order to constrain the progenitor scenario.
Superluminous Supernovae
Superluminous supernovae are an important class of object to understand, because they may be the few remaining examples of dying supermassive stars in low-metallicity environments, similar to the early universe. They are $\gtrsim$10 times more luminous than common SNe.
Hydrogen-poor SLSNe-I have a quite featureless early spectrum and are difficult to understand. One possible scenario that might explain such luminosities is that SLSNe-I are powered by an internal engine, such as a magnetar or accreting black hole. Strong magnetic fields or collimated jets can circularly polarize light. We tested the magnetar scenario for two SLSNe-I with circular polarimetry (Cikota+, in prep.), and did not detect any polarization. However, the magnetar scenario cannot be excluded as the powering engine.
Another approach to investigate SLSN progenitors is by studying their host galaxy environments. One hallmark of very massive progenitors would be a tendency to explode in very dense, UV-bright and blue regions. In Cikota+ 2017b, we investigate the spatially resolved host galaxy properties of two nearby hydrogen-poor SLSNe, using VLT/MUSE and HST data. The combination of high-resolution imaging and integral-field spectroscopy is very powerful in characterizing the explosion site. However, larger samples are needed to extract robust constraints on the progenitor population and how their galaxy environments affect the star formation process.
Large Interstellar Polarization Survey (LIPS)
I am part of the LIPS collaboration (PI: Nick Cox). In Bagnulo+ (2017), we present a sample of 109 stars observed in the southern hemisphere with VLT/FORS2. I am third author and contributed to the paper by (i) independently reducing a data subset to confirm the results; (ii) investigating the $\lambda_{max}$–$K$ relationship depending on the wavelength range by running a Monte Carlo simulation, and (iii) contributing to the discussion.
Other ongoing projects
Instruments calibration: I investigated the stability of VLT/FORS2 in spectropoalrimetry mode using a sample of standard stars (see Cikota+ 2017a). Furthermore, in the LIPS data, we found inconsistencies in polarization degree between successive observations with FORS2, in $\sim$10$\%$ of the data, which I aim to further investigate by means of my FORS2 calibration proposal that will be executed in P100, and by cross-matching the FORS2 data with William Herschel Telescope observations of common stars.
I also contributed to calibration of WFC3 grism spectra. The main motivation was to have an absolute color calibration in the UV-VIS-NIR with Hubble Space Telescope’s WFC3/IR data within 1$\%$ accuracy of 15 carefully selected stars, which is required by the Dark Energy science with JWST and other facilities (see e.g. the AAS abstract by Deustua, Cikota, et al. 2015).
ePESSTO SN survey: I am part of the extended-Public ESO Spectroscopic Survey for Transient Objects collaboration and $\sim$once a year go observing with the New Technology Telescope (NTT) at La Silla Observatory.
Interstellar medium in our Galaxy: I am part of a HST proposal to study the metallicity of the cool ISM in our own Galaxy, which was recently granted 26 orbits in cycle 25 (PI: De Cia) and plan to contribute in data reduction and analysis.
Dust around R Sculptoris: My VLT proposal to obtain spectropolarimetry of R Sculptoris will be executed in P100, with the aim to study dust around R Scu.
Small Solar System bodies & La Sagra Sky Survey
During my bachelor studies, from 2006-2009, I have been involved in discovering and tracking of Small Solar System Bodies at the La Sagra Observatory (OLS) in Andalusia, Spain. The first asteroids at OLS were discovered in August 2006 by manually “blinking” of sky images. In the following years I observed for over 300 nights (mostly remotely) and contributed to the continuous improvements of the search methods and data reduction pipelines, and co-discovered over 3100 minor bodies. In particular, some of the Python scripts and tools I have developed are:
(i) an observation planning tool with a graphical user interface, named “SkyPlot” (Figure 2);
(ii) a pipeline for an automatic detection of moving sources in a set of three or four images, their astrometry, and identification (the pipeline checks possible new minor planet suspects against known objects in the MPC Orbit database). If the candidate could not be identified, the script makes 3×3 croppies and uploads them in a html file;
(iii) an asteroids Night2night linking software which links observations of asteroids observed in multiple nights, and an asteroid prediction script, which predicts positions of asteroids and shows them in SkyPlot. The La Sagra Sky Survey (LSSS) has become the most prolific European NEO survey of all times.
At La Sagra Observatory, my brother Stefan and I started collaborating with Dr. Jose Luis Ortiz from the Instituto de Astrofisica de Andalucia (IAA-CSIC) on Trans-Neptunian Object and active Main Belt asteroid studies. We published a few works together, for instance:
“A mid-term astrometric and photometric study of trans-Neptunian object (90482) Orcus”
(Ortiz+ 2011), where we revealed the presence of Orcus’ satellite in our astrometric measurements. I am the second author and contributed by image reduction, astrometric measurements, orbit residuals calculation and first data analysis. Furthermore, I helped with the programming part and contributed to the discussion in “A photometric search for active Main Belt asteroids” (Cikota Stefan+ 2014), where I am third author.
Figure 2: Skyplot.