A reanalysis of the Gaia Data Release 2 photometric sensitivity curves using HST/STIS spectrophotometry

CONTEXT: The second Gaia data release (DR2) took place on April 2018. DR2 included photometry for more than 1.3 10^9 sources in G, BP, and RP. Even though Gaia DR2 photometry is very precise, there are currently three alternative definitions of the sensitivity curves that show significative differences. AIMS: The aim of this paper is to improve the quality of the input calibration data to produce new compatible definitions of the three bands and to identify the reasons for the discrepancies between previous definitions. METHODS: We have searched the HST archive for STIS spectra with G430L+G750L data obtained with wide apertures and combined them with the CALSPEC library to produce a high quality SED library of 122 stars with a broad range of colors, including three very red stars. We have used it to compute new sensitivity curves for G, BP, and RP using a functional analytical formalism. RESULTS: The new curves are significantly better than the two previous attempts, REV and WEI. For G we confirm the existence of a systematic bias in magnitude and correct a color term present in REV. For BP we confirm the need to define two magnitude ranges with different sensitivity curves and measure the cut between them at G = 10.87 with a significant increase in precision. The new curves also fit the data better than either REV or WEI. For RP we obtain a sensitivity curve that better fits the STIS spectra and we find that the differences with previous attempts reside in a systematic effect between ground-based and HST spectral libraries. Additional evidence from color-color diagrams indicate that the new sensitivity curve is more accurate. Nevertheless, there is still room for improvement in the accuracy of the sensitivity curves because of the current dearth of good-quality red calibrators: adding more to the sample should be a priority before Gaia data release 3 takes place.Comment: Accepted for publication in A&A, revised version with minor change

UV Extinction Towards a Quiescent Molecular Cloud in the SMC

Context: The mean UV extinction law for the Small Magellanic Cloud (SMC) is usually taken as a template for low-metallicity galaxies. However, its current derivation is based on only five stars, thus placing doubts on its universality. An increase in the number of targets with measured extinction laws in the SMC is necessary to determine its possible dependence on parameters such as metallicity and star-forming activity. Aims: To measure the UV extinction law for several stars in the quiescent molecular cloud SMC B1-1. Methods: We obtained HST/STIS slitless UV spectroscopy of a 25"x25" field of view and we combined it with ground-based NIR and visible photometry of the stars in the field. The results were processed using the Bayesian photometric package CHORIZOS to derive the visible-NIR extinction values for each star. The unextinguished Spectral Energy Distributions (SEDs) obtained in this way were then used to derive the UV extinction law for the four most extinguished stars. We also recalculated the visible-NIR extinction for the five SMC stars with preexisting UV extinction laws. Results: The UV extinction law for four SMC B1-1 stars within several pc of each other differs significantly from star to star. The 2175 {\AA} bump is moderately strong in one, weak in two, and absent in the fourth.Comment: 10 pages, 5 figures, accepted for publication in Astronomy & Astrophysic

An HST/NICMOS view of the prototypical giant HII region NGC604 in M33

We present the first high-spatial resolution near-infrared (NIR) imaging of NGC 604, obtained with the NICMOS camera aboard the Hubble Space Telescope (HST). These NICMOS broadband images reveal new NIR point sources, clusters, and diffuse structures. We found an excellent spatial correlation between the 8.4 GHz radio continuum and the 2.2mu-m nebular emission. Moreover, massive young stellar object candidates appear aligned with these radio peaks, reinforcing the idea that those areas are star-forming regions. Three different scaled OB associations are recognized in the NICMOS images. The brightest NIR sources in our images have properties that suggest that they are red supergiant stars, of which one of them was previously known. This preliminary analysis of the NICMOS images shows the complexity of the stellar content of the NGC 604 nebula.Comment: Paper presented in the Workshop "Young massive star clusters: initial conditions and environments" (Granada, Spain - Sept 2007). Astrophysics & Space Science in press, 7 pages, 4 figure

A reevaluation of the 2MASS zero points using CALSPEC spectrophotometry complemented with Gaia Data Release 2 parallaxes

CONTEXT. 2MASS is the reference survey in the NIR part of the spectrum given its whole-sky coverage, large dynamic range, and proven calibration uniformity. However, previous studies disagree in the value of the zero points (ZPs) for its three bands JHK at the hundredth of a magnitude level. The disagreement should become more noticeable now that Gaia provides whole-sky optical photometry calibrated below that level. AIMS. We want to establish the value of the 2MASS ZPs based on NICMOS/HST spectrophotometry of the CALSPEC standard stars and test it with the help of Gaia DR2 parallaxes. METHODS. We have computed the synthetic JHK photometry for a sample of stars using the HST CALSPEC spectroscopic standards and compared it with their 2MASS magnitudes to evaluate the ZPs. We have tested our results by analysing a sample of FGK dwarfs with excellent 2MASS photometry and accurate Gaia DR2 parallaxes.} RESULTS. The Vega ZPs for 2MASS J, H, and K are found to be -0.025$\pm$0.005 mag, 0.004$\pm$0.005 mag, and -0.015$\pm$0.005 mag, respectively. The analysis of the FGK sample indicates that the new ZPs are more accurate than previous ones.Comment: Accepted as a letter in A&A, 4 page

The infrared Hourglass cluster in M8

A detailed study of the Hourglass Nebula in the M8 star forming region is presented. The study is mainly based on recent subarcsec-resolution JHKs images taken at Las Campanas Observatory and complemented with archival HST images and longslit spectroscopy retrieved from the ESO Archive Facility. Using the new numerical code CHORIZOS, we estimate the distance to the earliest stars in the region to be 1.25 kpc. Infrared photometry of all the sources detected in the field is given. From analysis of the JHKs colour-colour diagrams, we find that an important fraction of these sources exhibit significant infrared excess. These objects are candidates to be low- and intermediate-mass pre-main sequence stars. Based on HST observations, the spatial distribution of gas, dust and stars in the region is analyzed. The morphological analysis of these images also reveals a rich variety of structures related to star formation (proplyds, jets, bow shocks), similar to those observed in M16 and M42, along with the detection of the first four Herbig-Haro objects in the region. Furthermore, a longslit spectrum obtained with NTT confirms the identification of one of them (HH 870) in the core of the Hourglass nebula, providing the first direct evidence of active star formation by accretion in M8.Comment: 22 pages, 15 figures, 3 tables, submitted to MNRAS. A preprint with high-resolution figures is available at http://www.dfuls.cl/~rbarba/arias_hourglass.pd

Broad Balmer Wings in BA Hyper/Supergiants Distorted by Diffuse Interstellar Bands: Five Examples in the 30 Doradus Region from the VLT-FLAMES Tarantula Survey

Extremely broad emission wings at Hβ and Hα have been found in VLT-FLAMES Tarantula Survey data for five very luminous BA supergiants in or near 30 Doradus in the Large Magellanic Cloud. The profiles of both lines are extremely asymmetrical, which we have found to be caused by very broad diffuse interstellar bands (DIBs) in the longward wing of Hβ and the shortward wing of Hα. These DIBs are well known to interstellar but not to many stellar specialists, so that the asymmetries may be mistaken for intrinsic features. The broad emission wings are generally ascribed to electron scattering, although we note difficulties for that interpretation in some objects. Such profiles are known in some Galactic hyper/supergiants and are also seen in both active and quiescent Luminous Blue Variables (LBVs). No prior or current LBV activity is known in these 30 Dor stars, although a generic relationship to LBVs is not excluded; subject to further observational and theoretical investigation, it is possible that these very luminous supergiants are approaching the LBV stage for the first time. Their locations in the HRD and presumed evolutionary tracks are consistent with that possibility. The available evidence for spectroscopic variations of these objects is reviewed, while recent photometric monitoring does not reveal variability. A search for circumstellar nebulae has been conducted, with an indeterminate result for one of them