Point Spread Function Fitting Research Articles

AIROPA IV: Validating point spread function reconstruction on various science cases

We present an analysis of six independent on-sky datasets taken with the Keck-II/NIRC2 instrument. Using the off-axis point spread function (PSF) reconstruction software AIROPA, we extract stellar astrometry, photometry, and other fitting metrics to characterize the performance of this package. We test the effectiveness of AIROPA to reconstruct the PSF across the field of view in varying atmospheric conditions, number and location of PSF reference stars, stellar crowding, and telescope position angle (PA). We compare the astrometric precision and fitting residuals between a static PSF model and a spatially varying PSF model that incorporates instrumental aberrations and atmospheric turbulence during exposures. Most of the fitting residuals we measure show little to no improvement in the variable-PSF mode over the single-PSF mode. For one of the data sets, we find photometric performance is significantly improved (by ∼10 × ) by measuring the trend seen in photometry as a function of off-axis location. For nearly all other metrics we find comparable astrometric and photometric precision across both PSF modes, with a ∼13 % smaller astrometric uncertainty in variable-PSF mode in the best case. We largely confirm that the spatially variable PSF does not significantly improve the astrometric and other PSF fitting residuals over the static PSF for on-sky observations. We attribute this to unaccounted instrumental aberrations that are not characterized through afternoon adaptive optics (AO) bench calibrations.

Demographics of the M-star Multiple Population in the Orion Nebula Cluster

We present updated results constraining multiplicity demographics for the stellar population of the Orion Nebula Cluster (ONC, a high-mass, high-density star-forming region), across primary masses 0.08–0.7 M ⊙. Our study utilizes archival Hubble Space Telescope data obtained with the Advanced Camera for Surveys using multiple filters (GO-10246). Previous multiplicity surveys in low-mass, low-density associations like Taurus identify an excess of companions to low-mass stars roughly twice that of the Galactic field and find the mass ratio distribution consistent with the field. Previously, we found the companion frequency to low-mass stars in the ONC is consistent with the Galactic field over mass ratios = 0.6–1.0 and projected separations = 30–160 au, without placing constraints on the mass ratio distribution. In this study, we investigate the companion population of the ONC with a double point-spread function (PSF) fitting algorithm sensitive to separations larger than 10 au (0.″025) using empirical PSF models. We identified 44 companions (14 new), and with a Bayesian analysis we estimate the companion frequency to low-mass stars in the ONC = 0.13 and the index of the power-law fit to the mass ratio distribution = 2.08 over all mass ratios and projected separations of 10–200 au. We find the companion frequency in the ONC is consistent with the Galactic field population, likely from states of high transient stellar density, and a probability of 0.002 that it is consistent with that of Taurus. We also find the ONC mass ratio distribution is consistent with the field and Taurus, potentially indicative of its primordial nature, a direct outcome of the star formation process.

Carina High-contrast Imaging Project for massive Stars (CHIPS)

Context.Most massive stars belong to multiple systems, yet the formation process leading to such high multiplicity remains insufficiently understood. To help constrain the different formation scenarios that exist, insights into the low-mass end of the companion mass function of such stars is crucial. However, this is a challenging endeavour as (sub-)solar mass companions at angular separations (ρ) below 1″ (corresponding to 1000–3000 au in nearby young open clusters and OB associations) are difficult to detect due to the large brightness contrast with the central star.Aims.With the Carina High-contrast Imaging Project of massive Stars (CHIPS), we aim to obtain statistically significant constraints on the presence and properties of low-mass companions around massive stars in a previously unreachable observing window (Δmag ≳ 10 atρ ≲ 1″). In the second paper of the series, we focus on the Trumpler 14 cluster, which harbours some of the youngest and most massive O-type stars in the Milky Way.Methods.We obtained VLT-SPHERE observations of seven O-type objects in Trumpler 14 using IRDIFS_EXT mode. These provide us with a 12″ × 12″ field of view (approximately ((3 × 3)×104au) centred on each O star and allow us to search for companions at separations larger than 0″​​.15 (approx. 360 au) and down to magnitude contrast > 10 mag in the near-infrared. We used angular and spectral differential imaging along with Point Spread Function (PSF) fitting to detect sources and measure their flux relative to that of the central object. We then used grids of ATLAS9 and PHOENIX Local Thermodynamic Equilibrium (LTE) atmosphere models combined with (pre-)main-sequence evolutionary tracks to estimate the mass of the detected candidate companions.Results.We detected 211 sources with near-infrared magnitude contrast in the range of 2–12. Given the large surface number density of stars in Trumpler 14, one cannot reliably distinguish between cluster members and genuine companions for most of the detected sources. The closest companion, at only 0″​​.26, is characterised as a 1.4M⊙star with an age of 0.6 Myr, in excellent agreement with previous age estimates for Tr 14. The mass function peaks at about 0.4M⊙and presents a dearth of stars in the 0.5–0.8M⊙mass range compared to previous estimates of the initial mass function in Tr 14. While statistically significant, part of these differences may result from contamination of theK-band fluxes by circumstellar material.Conclusions.SPHERE is clearly suitable to probe the low-mass end of the mass function in the vicinity of massive stars. Follow-up SPHERE observations to obtain the full Y to K spectral energy distribution would allow for better constraints on the masses of the detected sources, and to confirm (or invalidate) the curious mass function that we derived for low-mass stars in the vicinity of the O-type objects in Trumpler 14.

Binary Formation in the Orion Nebula Cluster: Exploring the Substellar Limit

Abstract We present results constraining the multiplicity of the very low mass stars and substellar objects in the Orion Nebula Cluster (ONC). Our sample covers primary masses 0.012–0.1 M ⊙ using archival Hubble Space Telescope data obtained with the Advanced Camera for Surveys using multiple filters. Studying the binary populations of clusters provides valuable constraints of how the birth environment affects binary formation and evolution. Prior surveys have shown that the binary populations of high-mass, high-density star clusters like the ONC may substantially differ from those in low-mass associations. Very low mass stellar and substellar binaries at wide separations, >20 au, are statistically rare in the Galactic field and have been identified in stellar associations like Taurus-Auriga and Ophiuchus. They also may be susceptible to dynamical interactions, and their formation may be suppressed by feedback from ongoing star formation. We implement a double point-spread function (PSF) fitting algorithm using empirical, position-dependent PSF models to search for binary companions at projected separations >10 au (0.″025). With this technique, we identify seven very low mass binaries, five of which are new detections, resulting in a binary frequency of 12 − 3.2 + 6 % over mass ratios of 0.5–1.0 and projected separations of 20–200 au. We find an excess of very low mass binaries in the ONC compared to the Galactic field, with a probability of 10−6 that the populations are statistically consistent. The substellar population of the ONC may require further dynamical processing of the lowest binding energy binaries to resemble the field population.

Simultaneous orientation and 3D localization microscopy with a Vortex point spread function

Estimating the orientation and 3D position of rotationally constrained emitters with localization microscopy typically requires polarization splitting or a large engineered Point Spread Function (PSF). Here we utilize a compact modified PSF for single molecule emitter imaging to estimate simultaneously the 3D position, dipole orientation, and degree of rotational constraint from a single 2D image. We use an affordable and commonly available phase plate, normally used for STED microscopy in the excitation light path, to alter the PSF in the emission light path. This resulting Vortex PSF does not require polarization splitting and has a compact PSF size, making it easy to implement and combine with localization microscopy techniques. In addition to a vectorial PSF fitting routine we calibrate for field-dependent aberrations which enables orientation and position estimation within 30% of the Cramér-Rao bound limit over a 66 μm field of view. We demonstrate this technique on reorienting single molecules adhered to the cover slip, λ-DNA with DNA intercalators using binding-activated localization microscopy, and we reveal periodicity on intertwined structures on supercoiled DNA.

Gaia Early Data Release 3

Context. The unprecedented astrometric precision of the Gaia mission relies on accurate estimates of the locations of sources in the Gaia data stream. This is ultimately performed by point spread function (PSF) fitting, which in turn requires an accurate reconstruction of the PSF, including calibrations of all the major dependences. These include a strong colour dependence due to Gaia’s broad G band and a strong time dependence due to the evolving contamination levels and instrument focus. Accurate PSF reconstruction is also important for photometry. Gaia Early Data Release 3 (EDR3) will, for the first time, use a PSF calibration that models several of the strongest dependences, leading to signficantly reduced systematic errors. Aims. We describe the PSF model and calibration pipeline implemented for Gaia EDR3, including an analysis of the calibration results over the 34 months of data. We include a discussion of the limitations of the current pipeline and directions for future releases. This will be of use both to users of Gaia data and as a reference for other precision astrometry missions. Methods. We develop models of the 1D line spread function (LSF) and 2D PSF profiles based on a linear combination of basis components. These are designed for flexibility and performance, as well as to meet several mathematical criteria such as normalisation. We fit the models to selected primary sources in independent time ranges, using simple parameterisations for the colour and other dependences. Variation in time is smoothed by merging the independent calibrations in a square root information filter, with resets at certain mission events that induce a discontinuous change in the PSF. Results. The PSF calibration shows strong time and colour dependences that accurately reproduce the varying state of the Gaia astrometric instrument. Analysis of the residuals reveals both the performance and the limitations of the current models and calibration pipeline, and indicates the directions for future development. Conclusions. The PSF modelling and calibration carried out for Gaia EDR3 represents a major step forwards in the data processing and will lead to reduced systematic errors in the core mission data products. Further significant improvements are expected in the future data releases.

Performing Point-spread Function Fitting with Photutils on Hubble Space Telescope Images

Abstract The low mass end of the Initial Mass Function (IMF) is a component of star formation that is not well understood. By analyzing Hubble Space Telescope (HST) images of star-forming regions, we aimed to provide data to constrain the shape of the low-mass IMF. We performed point-spread function (PSF) fitting using the Astropy Photutils package v0.6 on shallow HST Wide Field Camera 3 images taken in filters F110W, F139M, and F160W with the goal of identifying low mass stars and brown dwarfs. Residual images from our PSF fitting routine showed that fainter sources were effectively fit while fits to the brighter sources left residuals. Despite this, analysis confirmed that our PSF magnitudes are consistent with aperture magnitudes and mostly have smaller errors; 85%–100% of the flux was captured by the PSF fitting in most cases. PSF fitting of WFC3-IR images using Photutils shows promise for precise photometry.

A Full Implementation of Spectro-perfectionism for Precise Radial Velocity Exoplanet Detection: A Test Case With the MINERVA Reduction Pipeline

We present a computationally tractable implementation of spectro-perfectionism, a method which minimizes error imparted by spectral extraction. We develop our method in conjunction with a full raw reduction pipeline for the MINiature Exoplanet Radial Velocity Array (MINERVA), capable of performing both optimal extraction and spectro-perfectionism. Although spectro-perfectionism remains computationally expensive, our implementation can extract a MINERVA exposure in approximately 30 minutes. We describe our localized extraction procedure and our approach to point-spread function (PSF) fitting. We compare the performance of both extraction methods on a set of 119 exposures on HD 122064, an RV standard star. Both the optimal extraction and spectro-perfectionism pipelines achieve nearly identical RV precision under a six-exposure chronological binning. We discuss the importance of reliable calibration data for PSF fitting and the potential of spectro-perfectionism for future precise radial velocity exoplanet studies.

Searching for Wide Companions and Identifying Circum(sub)stellar Disks through PSF Fitting of Spitzer/IRAC Archival Images

Abstract Direct imaging surveys have discovered wide-orbit planetary-mass companions that challenge existing models of both star and planet formation, but their demographics remain poorly sampled. We have developed an automated binary companion point-spread function (PSF) fitting pipeline to take advantage of Spitzer's infrared sensitivity to planetary-mass objects and circum(sub)stellar disks, measuring photometry across the four Infrared Array Camera (IRAC) channels of 3.6, 4.5, 5.8, and 8.0 μm. We present PSF fitting photometry of archival Spitzer/IRAC images for 11 young, low-mass (M ∼ 0.044–0.88 M ⊙; K3.5–M7.5) members of three nearby star-forming regions (Chameleon, Taurus, and Upper Scorpius; d ∼ 150 pc; τ ∼ 1–10 Myr) that host confirmed or candidate faint companions at ρ = 1.″68–7.″31. We recover all system primaries, six confirmed, and two candidate low-mass companions in our sample. We also measure nonphotospheric [3.6]–[8.0] colors for three of the system primaries, four of the confirmed companions, and one candidate companion, signifying the presence of circumstellar or circum(sub)stellar disks. We furthermore report the confirmation of a ρ = 4.″66 (540 au) companion to [SCH06] J0359+2009 which was previously identified as a candidate via imaging over five years ago, but was not studied further. Based on its brightness (M [3.6] = 8.53 mag), we infer the companion mass to be M = 20 ± 5 M Jup given the primary’s model-derived age of 10 Myr. Our framework is sensitive to companions with masses less than 10 M Jup at separations of ρ = 300 au in nearby star-forming regions, opening up a new regime of parameter space that has yet to be studied in detail, discovering planetary-mass companions in their birth environments and revealing their circum(sub)stellar disks.

Insights into Kinesin-1 Stepping from Simulations and Tracking of Gold Nanoparticle-Labeled Motors

High-resolution tracking of gold nanoparticle-labeled proteins has emerged as a powerful technique for measuring the structural kinetics of processive enzymes and other biomacromolecules. These techniques use point spread function (PSF) fitting methods borrowed from single-molecule fluorescence imaging to determine molecular positions below the diffraction limit. However, compared to fluorescence, gold nanoparticle tracking experiments are performed at significantly higher frame rates and utilize much larger probes. In the current work, we use Brownian dynamics simulations of nanoparticle-labeled proteins to investigate the regimes in which the fundamental assumptions of PSF fitting hold and where they begin to break down. We find that because gold nanoparticles undergo tethered diffusion around their anchor point, PSF fitting cannot be extended to arbitrarily fast frame rates. Instead, camera exposure times that allow the nanoparticle to fully populate its stationary positional distribution achieve a spatial averaging that increases fitting precision. We furthermore find that changes in the rotational freedom of the tagged protein can lead to artifactual translations in the fitted particle position. Finally, we apply these lessons to dissect a standing controversy in the kinesin field over the structure of a dimer in the ATP waiting state. Combining new experiments with simulations, we determine that the rear kinesin head in the ATP waiting state is unbound but not displaced from its previous microtubule binding site and that apparent differences in separately published reports were simply due to differences in the gold nanoparticle attachment position. Our results highlight the importance of gold conjugation decisions and imaging parameters to high-resolution tracking results and will serve as a useful guide for the design of future gold nanoparticle tracking experiments.

Depth-dependent PSF calibration and aberration correction for 3D single-molecule localization.

Three-dimensional single molecule localization microscopy relies on the fitting of the individual molecules with a point spread function (PSF) model. The reconstructed images often show local squeezing or expansion in z. A common cause is depth-induced aberrations in conjunction with an imperfect PSF model calibrated from beads on a coverslip, resulting in a mismatch between measured PSF and real PSF. Here, we developed a strategy for accurate z-localization in which we use the imperfect PSF model for fitting, determine the fitting errors and correct for them in a post-processing step. We present an open-source software tool and a simple experimental calibration procedure that allow retrieving accurate z-positions in any PSF engineering approach or fitting modality, even at large imaging depths.

Insights into Kinesin-1 Stepping Dynamics from Brownian Dynamics Simulations and High-Resolution Tracking of Gold Nanoparticle-Labeled Motors

Abstract High-resolution tracking of gold nanoparticle-labeled proteins has emerged as a powerful technique for measuring the structural kinetics of processive enzymes and other biomacromolecules. These techniques use point-spread-function fitting methods borrowed from single-molecule fluorescence imaging to determine molecular positions below the diffraction limit. However, compared to fluorescence, gold nanoparticle tracking experiments are performed at significantly higher frame rates and utilize much larger probes. In the current work, we use Brownian dynamics simulations of nanoparticle-labeled proteins to investigate the regimes in which the fundamental assumptions of point-spread-function fitting hold and where they begin to break down. We find that because gold nanoparticles undergo tethered diffusion around their anchor point, point spread function fitting cannot be extended to arbitrarily fast frame rates. Instead, camera exposure times that allow the nanoparticle to fully populate its stationary positional distribution achieve a spatial averaging that increases fitting precision. We furthermore find that changes in the rotational freedom of the tagged protein can lead to artifactual translations in fitted particle position. Finally, we apply these lessons to dissect a standing controversy in the kinesin field over the structure of a dimer in the ATP waiting state. Combining new experiments with simulations, we determine that the rear kinesin head in the ATP waiting state is unbound but not displaced from its previous microtubule binding site, and that apparent differences in separately published reports were simply due to differences in the gold nanoparticle attachment position. Our results highlight the importance of gold conjugation decisions and imaging parameters to high-resolution tracking results, and will serve as a useful guide for the design of future gold nanoparticle tracking experiments.

3D Super Resolution Imaging Reveals that Plasma Membrane Topology Can Be Misinterpreted as Lateral Heterogeneity in Cells Imaged Under Total Internal Reflection

Total internal reflection (TIR) fluorescence microscopy is a powerful imaging technique in which fluorescence illumination occurs within ∼100nm of a glass-water interface, enabling selective excitation of fluorophores in the ventral plasma membrane of adherent cells. TIR illumination is often used in conjunction with localization based super-resolution imaging methods (STORM and PALM) because it dramatically reduces fluorescence from probes and background originating from positions away from the glass surface. Past work in the Veatch lab has used super-resolution imaging in conjunction with TIR illumination to quantify the concentration and colocalization of probes embedded within the plasma membrane of adherent cells. The goals of the current work is to use 3 dimensional (3D) super-resolution microscopy to characterize the three dimensional shape of the ventral plasma membrane of several cell types and to explore how TIR or off-TIR illumination impacts the apparent concentration of membrane probes imaged at the cell surface. Measurements quantifying 3D membrane topology are conducted using double helix point spread function (PSF) along with arbitrary PSF fitting algorithms to generate reconstructed super-resolution images with resolution of 15nm in x-y and ∼20nm the z dimension. For some cell types the membrane to coverslip distance varied over 100nm with regions closer to the glass appearing brighter in conventional TIR imaging and generating a higher density of localizations in super-resolution imaging. A more uniform apparent surface density of probes was observed by both imaging methods when membranes were illuminated outside of TIR. We conclude that the height dependent illumination intensity within the TIR field is responsible for reducing the density of probes detected from membrane regions localized further from the glass surface. Ongoing efforts to address this issue will be discussed.

31 EPID response modeling with Monte Carlo for Novalis TrueBeam STx

Introduction Portal Dosimetry is a very convenient tool to perform patient QA for VMAT plan with TrueBeam STx Linac. On the other hand, it is not sensitive to gantry rotation. Monte Carlo modeling of EPID response could offer an alternative to the PDIP (portal dosimetry prediction algorithm) in Eclipse. It also opens wider perspectives: response prediction with phantom or patient. Preliminary work focuses on PSF (point spread function) fitting, and verification against measurements. Methods As we cannot irradiate EPID with a very narrow ( Monte Carlo calculation of the EPID response is performed with PenEasy. A Phase Space is obtained after collimator from PRIMO. This phase space can then be handled by PenEasy (including EPID spread function), with simple object in the beam path. Results The sharpest transition from blocked to open beam is obtained using metal blocks behind EPID. A.Latorre showed that best fit to experimental results is obtained with modified Lorentzian model (with FWHM = 0.74 mm). This model fits well the steepest part of the penumbra. Minor deviation ( 5%) occurs in the furthest region under open beam. This model does not fit under the blocked area (probably due to block transmission/scattering). Ongoing step is to measure EPID response with simple regular shaped PMMA blocks. Conclusions Monte Carlo simulation (combining PRIMO, PenEasy and fitted EPID spread function) offers the perspective of predicting EPID response for TrueBeam STx linac. It can be used as an alternative to PDIP algorithm. Using it with anisotropic phantom could make Portal Dosimetry sensitive to gantry rotation. Using it in vivo may allow to check patient positioning against CT positioning in TPS.

Simultaneously Measuring Image Features and Resolution in Live-Cell STED Images

Reliable interpretation and quantification of cellular features in fluorescence microscopy requires an accurate estimate of microscope resolution. This is typically obtained by measuring the image of a nonbiological proxy for a point-like object, such as a fluorescent bead. Although appropriate for confocal microscopy, bead-based measurements are problematic for stimulated emission depletion microscopy and similar techniques where the resolution depends critically on the choice of fluorophore and acquisition parameters. In this article, we demonstrate that for a known geometry (e.g., tubules), the resolution can be measured in situ by fitting a model that accounts for both the point spread function (PSF) and the fluorophore distribution. To address the problem of coupling between tubule diameter and PSF width, we developed a technique called nested-loop ensemble PSF fitting. This approach enables extraction of the size of cellular features and the PSF width in fixed-cell and live-cell images without relying on beads or precalibration. Nested-loop ensemble PSF fitting accurately recapitulates microtubule diameter from stimulated emission depletion images and can measure the diameter of endoplasmic reticulum tubules in live COS-7 cells. Our algorithm has been implemented as a plugin for the PYthon Microscopy Environment, a freely available and open-source software.

Dust Production Rates in the Fomalhaut Debris Disk from SOFIA/FORCAST Mid-infrared Imaging

Abstract We present the first spatially resolved mid-infrared (37.1 μm) image of the Fomalhaut debris disk. We use point spread function fitting and subtraction to distinctly measure the flux from the unresolved component and the debris disk. We measure an infrared excess in the point source of 0.9 ± 0.2 Jy, consistent with emission from warm dust in an inner disk structure, and inconsistent with a stellar wind origin. We cannot confirm or rule out the presence of a pileup ring near the star. In the cold region, the 37 μm imaging is sensitive to emission from small, blowout grains, which is an excellent probe of the dust production rate from planetesimal collisions. Under the assumptions that the dust grains are icy aggregates and the debris disk is in steady state, this result is consistent with the dust production rates predicted by Kenyon & Bromley from theoretical models of icy planet formation. We find a dust luminosity of (7.9 ± 0.8) × 10−4 L ⊙ and a dust mass of 8–16 lunar masses, depending on grain porosity, with ∼1 lunar mass in grains with radius 1 μm–1 mm. If the grains are icy and highly porous, meter-sized objects must be invoked to explain the far-infrared, submm, and mm emission. If the grains are composed of astronomical silicates, there is a dearth of blowout grains and the mass loss rate is well below the predicted dust production values.

Automated clustering method for point spread function classification

The point spread function (PSF) plays a very important part in image post-processing and high-precion astrometry and photometry. It is necessary to analyse the properties of the PSF before we use it to process data. However, in real observations, the PSF is affected by many different factors and the shape of it has inevitable spatial and temporal variations that can be hard to describe. In this paper, we propose a clustering method to evaluate the shape variations of PSFs. We analyse the performance of this method with simulated PSFs under different observation conditions. Then, we process two observational data sets with this method. The PSF clustering results can provide a reference for checking observation conditions and can be used for astrometry based on PSF fitting. In general, our method can reveal the morphologic similarities of different PSFs and can provide a reference for observations. The cluster revealed by our method can provide a reference for the evaluation of observation conditions and for the post-processing of astronomical observation data.

Point-spread Function Ramifications and Deconvolution of a Signal Dependent Blur Kernel Due to Interpixel Capacitive Coupling

Interpixel capacitance (IPC) is a deterministic electronic coupling that results in a portion of the collected signal incident on one pixel of a hybridized detector array being measured in adjacent pixels. Data collected by light sensitive HgCdTe arrays which exhibit this coupling typically goes uncorrected or is corrected by treating the coupling as a fixed point-spread function. Evidence suggests that this IPC coupling is not uniform across different signal and background levels. This variation invalidates assumptions that are key in decoupling techniques such as Wiener Filtering or application of the Lucy–Richardson algorithm. Additionally, the variable IPC results in the point-spread function (PSF) depending upon a star’s signal level relative to the background level, among other parameters. With an IPC ranging from 0.68% to 1.45% over the full well depth of a sensor, as is a reasonable range for the H2RG arrays, the FWHM of the JWSTs NIRCam 405N band is degraded from 2.080 pix (0.″132) as expected from the diffraction pattern to 2.186 pix (0.″142) when the star is just breaching the sensitivity limit of the system. For example, When attempting to use a fixed PSF fitting (e.g., assuming the PSF observed from a bright star in the field) to untangle two sources with a flux ratio of 4:1 and a center to center distance of 3 pixels, flux estimation can be off by upwards of 1.5% with a separation error of 50 millipixels. To deal with this issue an iterative non-stationary method for deconvolution is here proposed, implemented, and evaluated that can account for the signal dependent nature of IPC.

Uncrowding R 136 from VLT/SPHERE extreme adaptive optics

This paper presents the sharpest near-IR images of the massive cluster R 136 to date, based on the extreme adaptive optics of the SPHERE focal instrument implemented on the ESO Very Large Telescope and operated in its IRDIS imaging mode.The crowded stellar population in the core of the R 136 starburst compact cluster remains still to be characterized in terms of individual luminosities, age, mass and multiplicity. SPHERE/VLT and its high contrast imaging possibilities open new windows to make progress on these questions.Stacking-up a few hundreds of short exposures in J and Ks spectral bands over a field of view (FoV) of 10.9″ × 12.3″ centered on the R 136a1 stellar component, enabled us to carry a refined photometric analysis of the core of R 136. We detected 1110 and 1059 sources in J and Ks images respectively with 818 common sources. Thanks to better angular resolution and dynamic range, we found that more than 62.6% (16.5%) of the stars, detected both in J and Ks data, have neighbours closer than 0.2′′ (0.1′′). The closest stars are resolved down to the full width at half maximum (FWHM) of the point spread function (PSF) measured by Starfinder. Among resolved and/or detected sources R 136a1 and R 136c have optical companions and R 136a3 is resolved as two stars (PSF fitting) separated by 59 ± 2 mas. This new companion of R 136a3 presents a correlation coefficient of 86% in J and 75% in Ks. The new set of detected sources were used to re-assess the age and extinction of R 136 based on 54 spectroscopically stars that have been recently studied with HST slit-spectroscopy (Crowther et al. 2016, MNRAS, 458, 624) of the core of this cluster. Over 90% of these 54 sources identified visual companions (closer than 0.2′′). We found the most probable age and extinction for these sources are 1.8+1.2-0.8 Myr, AJ = (0.45 ± 0.5) mag and AK = (0.2 ± 0.5) mag within the photometric and spectroscopic error-bars. Additionally, using PARSEC evolutionary isochrones and tracks, we estimated the stellar mass range for each detected source (common in J and K data) and plotted the generalized histogram of mass (MF with error-bars). Using SPHERE data, we have gone one step further and partially resolved and studied the initial mass function covering mass range of (3–300) M⊙ at the age of 1 and 1.5 Myr. The density in the core of R 136 (0.1–1.4 pc) is estimated and extrapolated in 3D and larger radii (up to 6 pc). We show that the stars in the core are still unresolved due to crowding, and the results we obtained are upper limits. Higher angular resolution is mandatory to overcome these difficulties.

SPITZER PHOTOMETRY OF ∼1 MILLION STARS IN M31 AND 15 OTHER GALAXIES*

ABSTRACT We present Spitzer IRAC 3.6–8 μm and Multiband Imaging Photometer 24 μm point-source catalogs for M31 and 15 other mostly large, star-forming galaxies at distances ∼3.5–14 Mpc, including M51, M83, M101, and NGC 6946. These catalogs contain ∼1 million sources including ∼859,000 in M31 and ∼116,000 in the other galaxies. They were created following the procedures described in Khan et al. through a combination of point-spread function (PSF) fitting and aperture photometry. These data products constitute a resource to improve our understanding of the IR-bright (3.6–24 μm) point-source populations in crowded extragalactic stellar fields and to plan observations with the James Webb Space Telescope.