9 research outputs found
Spectral classification of young stars using conditional invertible neural networks I. Introducing and validating the method
Aims. We introduce a new deep learning tool that estimates stellar parameters
(such as effective temperature, surface gravity, and extinction) of young
low-mass stars by coupling the Phoenix stellar atmosphere model with a
conditional invertible neural network (cINN). Our networks allow us to infer
the posterior distribution of each stellar parameter from the optical spectrum.
Methods. We discuss cINNs trained on three different Phoenix grids: Settl,
NextGen, and Dusty. We evaluate the performance of these cINNs on unlearned
Phoenix synthetic spectra and on the spectra of 36 Class III template stars
with well-characterised stellar parameters.
Results. We confirm that the cINNs estimate the considered stellar parameters
almost perfectly when tested on unlearned Phoenix synthetic spectra. Applying
our networks to Class III stars, we find good agreement with deviations of at
most 5--10 per cent. The cINNs perform slightly better for earlier-type stars
than for later-type stars like late M-type stars, but we conclude that
estimations of effective temperature and surface gravity are reliable for all
spectral types within the network's training range.
Conclusions. Our networks are time-efficient tools applicable to large
amounts of observations. Among the three networks, we recommend using the cINN
trained on the Settl library (Settl-Net), as it provides the best performance
across the largest range of temperature and gravity.Comment: 29 pages, 19 figures, Accepted for publication by Astronomy &
Astrophysics on 10. Apri
Noise-Net: Determining physical properties of HII regions reflecting observational uncertainties
Stellar feedback, the energetic interaction between young stars and their
birthplace, plays an important role in the star formation history of the
universe and the evolution of the interstellar medium (ISM). Correctly
interpreting the observations of star-forming regions is essential to
understand stellar feedback, but it is a non-trivial task due to the complexity
of the feedback processes and degeneracy in observations. In our recent paper,
we introduced a conditional invertible neural network (cINN) that predicts
seven physical properties of star-forming regions from the luminosity of 12
optical emission lines as a novel method to analyze degenerate observations. We
demonstrated that our network, trained on synthetic star-forming region models
produced by the WARPFIELD-Emission predictor (WARPFIELD-EMP), could predict
physical properties accurately and precisely. In this paper, we present a new
updated version of the cINN that takes into account the observational
uncertainties during network training. Our new network named Noise-Net reflects
the influence of the uncertainty on the parameter prediction by using both
emission-line luminosity and corresponding uncertainties as the necessary input
information of the network. We examine the performance of the Noise-Net as a
function of the uncertainty and compare it with the previous version of the
cINN, which does not learn uncertainties during the training. We confirm that
the Noise-Net outperforms the previous network for the typical observational
uncertainty range and maintains high accuracy even when subject to large
uncertainties.Comment: 22 pages, 14 figures, Accepted for publication by MNRAS on 04.
Januar
Exoplanet characterization using conditional invertible neural networks
The characterization of an exoplanet's interior is an inverse problem, which
requires statistical methods such as Bayesian inference in order to be solved.
Current methods employ Markov Chain Monte Carlo (MCMC) sampling to infer the
posterior probability of planetary structure parameters for a given exoplanet.
These methods are time consuming since they require the calculation of a large
number of planetary structure models. To speed up the inference process when
characterizing an exoplanet, we propose to use conditional invertible neural
networks (cINNs) to calculate the posterior probability of the internal
structure parameters. cINNs are a special type of neural network which excel in
solving inverse problems. We constructed a cINN using FrEIA, which was then
trained on a database of internal structure models to recover
the inverse mapping between internal structure parameters and observable
features (i.e., planetary mass, planetary radius and composition of the host
star). The cINN method was compared to a Metropolis-Hastings MCMC. For that we
repeated the characterization of the exoplanet K2-111 b, using both the MCMC
method and the trained cINN. We show that the inferred posterior probability of
the internal structure parameters from both methods are very similar, with the
biggest differences seen in the exoplanet's water content. Thus cINNs are a
possible alternative to the standard time-consuming sampling methods. Indeed,
using cINNs allows for orders of magnitude faster inference of an exoplanet's
composition than what is possible using an MCMC method, however, it still
requires the computation of a large database of internal structures to train
the cINN. Since this database is only computed once, we found that using a cINN
is more efficient than an MCMC, when more than 10 exoplanets are characterized
using the same cINN.Comment: 15 pages, 13 figures, submitted to Astronomy & Astrophysic
A deep learning approach for the 3D reconstruction of dust density and temperature in star-forming regions
Funding: The team in Heidelberg acknowledges funding from the European Research Council via the ERC Synergy Grant “ECOGAL” (project ID 855130), from the German Excellence Strategy via the Heidelberg Cluster of Excellence (EXC 2181 - 390900948) “STRUCTURES”, and from the German Ministry for Economic Affairs and Climate Action in project “MAINN” (funding ID 50OO2206). They also thank for computing resources provided by The Länd and DFG through grant INST 35/1134-1 FUGG and for data storage at SDS@hd through grant INST 35/1314-1 FUGG.Aims. We introduce a new deep learning approach for the reconstruction of 3D dust density and temperature distributions from multi-wavelength dust emission observations on the scale of individual star-forming cloud cores (< 0.2 pc). Methods. We construct a training data set by processing cloud cores from the Cloud Factory simulations with the POLARIS radiative transfer code to produce synthetic dust emission observations at 23 wavelengths between 12 and 1300 µm. We simplify the task by reconstructing the cloud structure along individual lines of sight and train a conditional invertible neural network (cINN) for this purpose. The cINN belongs to the group of normalising flow methods and is able to predict full posterior distributions for the target dust properties. We test different cINN setups, ranging from a scenario that includes all 23 wavelengths down to a more realistically limited case with observations at only seven wavelengths. We evaluate the predictive performance of these models on synthetic test data. Results. We report an excellent reconstruction performance for the 23-wavelengths cINN model, achieving median absolute relative errors of about 1.8% in log(ndust/m−3) and 1% in log(Tdust/K), respectively. We identify trends towards overestimation at the low end of the density range and towards underestimation at the high end of both density and temperature, which may be related to a bias in the training data. Limiting coverage to a combination of only seven wavelengths, we still find a satisfactory performance with average absolute relative errors of about 3.3% and 2.5% in log(ndust/m−3) and log(Tdust/K). Conclusions. This proof of concept study shows that the cINN-based approach for 3D reconstruction of dust density and temperature is very promising and even feasible under realistic observational constraints.Peer reviewe
Measuring Young Stars in Space and Time -- II. The Pre-Main-Sequence Stellar Content of N44
The Hubble Space Telescope (HST) survey Measuring Young Stars in Space and
Time (MYSST) entails some of the deepest photometric observations of
extragalactic star formation, capturing even the lowest mass stars of the
active star-forming complex N44 in the Large Magellanic Cloud. We employ the
new MYSST stellar catalog to identify and characterize the content of young
pre-main-sequence (PMS) stars across N44 and analyze the PMS clustering
structure. To distinguish PMS stars from more evolved line of sight
contaminants, a non-trivial task due to several effects that alter photometry,
we utilize a machine learning classification approach. This consists of
training a support vector machine (SVM) and a random forest (RF) on a carefully
selected subset of the MYSST data and categorize all observed stars as PMS or
non-PMS. Combining SVM and RF predictions to retrieve the most robust set of
PMS sources, we find candidates with a PMS probability above 95%
across N44. Employing a clustering approach based on a nearest neighbor surface
density estimate, we identify 18 prominent PMS structures at
significance above the mean density with sub-clusters persisting up to and
beyond significance. The most active star-forming center, located
at the western edge of N44's bubble, is a subcluster with an effective radius
of pc entailing more than 1,100 PMS candidates. Furthermore, we
confirm that almost all identified clusters coincide with known H II regions
and are close to or harbor massive young O stars or YSOs previously discovered
by MUSE and Spitzer observations.Comment: 29 pages, 21 figures, accepted for publication in A
Measuring Young Stars in Space and Time -- I. The Photometric Catalog and Extinction Properties of N44
In order to better understand the role of high-mass stellar feedback in
regulating star formation in giant molecular clouds, we carried out a Hubble
Space Telescope (HST) Treasury Program "Measuring Young Stars in Space and
Time" (MYSST) targeting the star-forming complex N44 in the Large Magellanic
Cloud (LMC). Using the F555W and F814W broadband filters of both the ACS and
WFC3/UVIS, we built a photometric catalog of 461,684 stars down to
mag and mag,
corresponding to the magnitude of an unreddened 1 Myr pre-main-sequence star of
at the LMC distance. In this first paper we describe
the observing strategy of MYSST, the data reduction procedure, and present the
photometric catalog. We identify multiple young stellar populations tracing the
gaseous rim of N44's super bubble, together with various contaminants belonging
to the LMC field population. We also determine the reddening properties from
the slope of the elongated red clump feature by applying the machine learning
algorithm RANSAC, and we select a set of Upper Main Sequence (UMS) stars as
primary probes to build an extinction map, deriving a relatively modest median
extinction mag. The same procedure applied to
the red clump provides mag.Comment: 29 pages, 15 figures, accepted for publication in A
Hubble Tarantula Treasury Project - VI. Identification of Pre-Main-Sequence Stars using Machine Learning techniques
The Hubble Tarantula Treasury Project (HTTP) has provided an unprecedented
photometric coverage of the entire star-burst region of 30 Doradus down to the
half Solar mass limit. We use the deep stellar catalogue of HTTP to identify
all the pre--main-sequence (PMS) stars of the region, i.e., stars that have not
started their lives on the main-sequence yet. The photometric distinction of
these stars from the more evolved populations is not a trivial task due to
several factors that alter their colour-magnitude diagram positions. The
identification of PMS stars requires, thus, sophisticated statistical methods.
We employ Machine Learning Classification techniques on the HTTP survey of more
than 800,000 sources to identify the PMS stellar content of the observed field.
Our methodology consists of 1) carefully selecting the most probable low-mass
PMS stellar population of the star-forming cluster NGC 2070, 2) using this
sample to train classification algorithms to build a predictive model for PMS
stars, and 3) applying this model in order to identify the most probable PMS
content across the entire Tarantula Nebula. We employ Decision Tree, Random
Forest and Support Vector Machine classifiers to categorise the stars as PMS
and Non-PMS. The Random Forest and Support Vector Machine provided the most
accurate models, predicting about 20,000 sources with a candidateship
probability higher than 50 percent, and almost 10,000 PMS candidates with a
probability higher than 95 percent. This is the richest and most accurate
photometric catalogue of extragalactic PMS candidates across the extent of a
whole star-forming complex.Comment: 26 pages, 25 figures, Accepted for publication in MNRA
Characterising Pre-Main-Sequence Stars in the Large Magellanic Cloud with Machine and Deep Learning Techniques
The Large Magellanic Cloud (LMC) exhibits an extraordinary star-forming activity, providing excellent targets for star formation research. Photometric observations with the Hubble Space Telescope (HST) allow for deep, high-resolution studies of young stellar clusters and still-forming pre-main-sequence (PMS) stars in the LMC. In this thesis we study two LMC star-forming complexes, the Tarantula Nebula and N44. Using HST photometry of the Tarantula Nebula from the "Hubble Tarantula Treasury Project" (HTTP), we devise a machine-learning (ML) classification procedure to identify PMS stars from photometry and recover the PMS population captured by the HTTP survey. We introduce new HST observations of N44, the "Measuring Young Stars in Space and Time" (MYSST) survey, identify N44’s PMS content with our ML classification procedure, and conduct a clustering analysis of the identified PMS stars. Additionally, we develop a conditional invertible neural network approach to predict stellar physical parameters from photometric observations, based on the PARSEC stellar evolution models. We perform a test on HST observations of the Milky Way clusters Westerlund 2 and NGC 6397, and successfully confirm previous findings on e.g. the age of Westerlund 2. For NGC 6397, however, we identify discrepancies between the PARSEC stellar evolution models and HST observations that prevent accurate predictions