J/A+A/706/A312 LkCa 15 polarimetric images (Swastik+, 2026)
Imaging the LkCa 15 system in polarimetry and total intensity without
self-subtraction artefacts.
Swastik C., Wahhaj Z., Benisty M., Arora S., Ginski C., Ren B.B.,
van Holstein R.G., de Rosa R., Banyal R.K., Tazaki R.
<Astron. Astrophys. 706, A312 (2026)>
=2026A&A...706A.312S 2026A&A...706A.312S (SIMBAD/NED BibCode)
ADC_Keywords: YSOs ; Polarization ; Optical
Keywords: protoplanetary disks - planet-disk interactions -
stars: pre-main sequence - stars: variables: T Tauri, Herbig Ae/Be
Abstract:
Studying young protoplanetary disks is essential for understanding
planet formation, but traditional angular differential imaging
introduces self-subtraction artefacts that hinder the interpretation
of small-scale disk structures. We present high-resolution Ks-band
total- and polarized- intensity images of the LkCa 15 system that are
free of such artefacts.
LkCa 15 is a young proto-planetary system with a ∼160au disc and
previous claims of two protoplanet candidates at 15 and 18au. We aim
to analyse the LkCa 15 proto-planetary disc using high-contrast
imaging to search for super-Jupiter planets beyond 20au and to
characterise the dust distribution and grain composition.
We used near-simultaneous reference-star differential imaging (RDI,
"star-hopping") to obtain self-subtraction-free Ks-band images
beyond 0.1". We first modelled the Ks-band total- and
polarised-intensity images together with ALMA submillimetre continuum
maps using RADMC-3D and a two grain-size (micron and millimetre)
compact olivine model. Residual mismatches in the near-IR then
motivated us to extract the scattering phase function, S(θ),
and polarised fraction, P(θ), from the SPHERE data and compare
them with aggregate-scattering models, which pointed to porous CAHP
grains in the surface layer and led us to recompute the NIR
scattered-light models with CAHP.
Our initial two grain-size (micron and millimetre) olivine model
roughly reproduces the observed NIR and ALMA disc morphology, with a
flared micron surface layer from ∼25-85au (H/R∼0.08 at 50au;
surface gap ∼35-40au) and a millimetre mid-plane ring from ∼55-130au
with a gap at ∼75-100au, for i∼50° and PA∼61°. The near-IR
data, however, are less forward-scattering than the model. From the
phase functions, we find that S(θ) rises by ∼5x from
θ∼90° to θ∼35°, while P(θ) shows a broad
sub-Rayleigh peak with Pmax∼0.35 near θ∼90°. These
analyses disfavour compact olivine Mie spheres and are better matched
by porous aggregates (CAHP-128-100nm), so we recomputed the NIR
scattered-light models with CAHP-128-100nm grains in the surface
layer (retaining compact millimetre grains for the ALMA continuum),
which improves the match to the Ks-band morphology and polarisation.
From the number ratio between the 12um and 2mm grains, we inferred
a size-distribution slope of {dzeta}~-2.3. Although no new candidate
planets were detected, we estimated upper mass limits: beyond 200au,
planets more massive than ∼1.5MJ are unlikely, while in the inner
disc planets up to ∼3.6MJ could remain undetected.
The star-hopping RDI data, together with phase-function diagnostics
and RADMC-3D modelling with compact olivine and porous CAHP grains,
allow us to reproduce the main observed features of the LkCa 15
system. The number ratio between the 12um and 2mm olivine grains
further shows that micron-sized grains are under-abundant relative to
size distributions in the ISM or debris discs, providing new insights
into grain growth and dust dynamics in gas-rich proto-planetary discs.
Description:
The data were obtained with VLT/SPHERE-IRDIS
using reference-star differential imaging (star-hopping). The released
data include calibrated Stokes I, Q, and U images suitable for disk
morphology and dust-scattering studies.
The data files supplied here consist of science-ready FITS images
presented in the paper. Observations were obtained with
VLT/SPHERE-IRDIS in the Ks band using the polarimetric mode and the
star-hopping reference-star differential imaging technique. Each file
contains a two-dimensional image array with north up and east left.
The images were derotated, corrected for instrumental polarisation,
and aligned to a common orientation. Basic WCS information and image
dimensions are provided in the FITS headers.
The Stokes I images contain the total intensity data. LkCa15_I.fits
is flux-calibrated in Jy, while LkCa15IADU.fits preserves the raw
detector analogue-to-digital units after median combination of the
two IRDIS detector channels.
The Stokes Q and U images contain the linearly polarised signal.
All images are centred on LkCa 15. The stellar position is given by
the CRPIX keywords in the FITS headers.
North is up and east is left in all frames.
The data may be analysed directly using standard FITS tools.
Objects:
--------------------------------------------------------
RA (2000) DE Designation(s)
--------------------------------------------------------
04 39 17.79 +22 21 03.3 LkCa 15 = IRAS 04363+2215
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File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 169 4 List of fits files
fits/* . 4 Individual fits files
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See also:
J/A+A/639/A121 : LkCa 15 and 2MASS J16100501-2132318 ALMa images
(Facchini+, 2020)
J/A+A/670/A12 : Integrated spectra and moment 0 maps of LkCa 15 (Sturm+, 2023)
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 h RAh Right Ascension of center (J2000)
4- 5 I2 min RAm Right Ascension of center (J2000)
7- 11 F5.2 s RAs Right Ascension of center (J2000)
12 A1 --- DE- Declantion sign of center (J2000)
13- 14 I2 deg DEd Declination of center (J2000)
16- 17 I2 arcmin DEm Declination of center (J2000)
19- 22 F4.1 arcsec DEs Declination of center (J2000)
24- 27 I4 --- Nx Number of pixels along X-axis
29- 32 I4 --- Ny Number of pixels along Y-axis
34- 37 I4 Kibyte size Size of FITS file
39- 55 A17 --- FileName Name of FITS file, in subdirectory fits
57-169 A113 --- Title Title of the FITS file
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Acknowledgements:
Swastik Chowbay, swastik.chowbay(at)unimi.it
(End) Patricia Vannier [CDS] 26-Dec-2025