Aleksandar Cikota’s
RESEARCH EXPERIENCE
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 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.
Figure 1: Both, SNe Ia and PPNe, have steeply ris-
ing polarization curves towards blue wavelengths with
λ
max
. 0.4 µm, compared to HD 141318, a normal
Galactic star with λ
max
0.55 µm. Source: Cikota+
2017c.
SPECTROPOLARIMETRY OF SUPERNOVAE Ia
Understanding the effect of dust extinction
on SNe Ia magnitudes is essential for accu-
rate measurement of cosmological parame-
ters and the expansion history of the Uni-
verse. Studying dust along sight lines to SNe
Ia might also lead to conclusions on the pro-
genitor systems, because different progeni-
tor systems imply different circumstellar en-
vironments. Intriguingly, studies of the host-
galaxy dust extinction from SNe Ia yielded
diverse values of the total-to-selective extinc-
tion ratio, often R
V
. 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 et
al. 2016).
Furthermore, spectropolarimetry is a pow-
erful technique that enables us to study in-
terstellar 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, λ
max
, depends on the dust grain size
distribution (Serkowski et al. 1975). 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 (λ
max
. 0.4µm, Patat et al. 2015), which
in general implies an enhanced abundance of small dust grains. For comparison, normal sight lines
to Milky Way stars have λ
max
0.55µm. 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.
1
However, we found that they have normal polarization curves (Cikota et al., 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 et
al. 2005). Thus, we suggest that also some SNe Ia polarization curves might be produced by CSM
dust scattering (Cikota et al. 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 Stock-
holm), 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 &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 et al., 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 et al. 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 et al. (submitted), 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 λ
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 et al. 2017a). Furthermore, in the LIPS data, we found
inconsistencies in polarization degree between successive observations with FORS2, in 10% of the
2
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 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.
Figure 2: Skyplot planning tool
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 Au-
gust 2006 by manually ”blinking” of sky im-
ages. 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 particu-
lar, some of the Python scripts and tools I have
developed are: (i) an observation planning tool
with a graphical user interface, named ”Sky-
Plot” (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 identi-
fied, the script makes 3x3 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 astro-
metric and photometric study of trans-Neptunian object (90482) Orcus” (Ortiz et al. 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
3
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 et al. 2014),
where I am third author.
References: Cikota A.+ 2016, ApJ, 819, 152 Cikota A.+ 2017a, MNRAS, 464, 4146 Cikota A.+
2017b, MNRAS, 469, 4705 Cikota A.+ 2017c, MNRAS, 471, 2111 Cikota S.+ 2014, A&A, 562, 94
Deustua S.+, 2015, IAUGA, 2258218 Kennicutt R. C.+ 2011, PASP, 123, 1347 Kashi, A., & Soker, N.
2011, MNRAS, 417, 1466 Klochkova V. G.+ 2016, Astronomy Reports, 60, 344 Oppenheimer B. D.+
2005, ApJ, 624, 957 Ortiz J.-L.+ 2011, A&A, 525, 31 Patat F.+, 2007, Science, 317, 924 Patat F.+
2015, A&A, 577, A53 Serkowski K.+ 1975, ApJ, 196, 261
4