Total to Selective Extinction Ratios and Visual Extinctions from Ultraviolet Data

We present determinations of the total to selective extinction ratio R_V and visual extinction A_V values for Milky Way stars using ultraviolet color excesses. We extend the analysis of Gnacinski and Sikorski (1999) by using non-equal weights derived from observational errors. We present a detailed discussion of various statistical errors. In addition, we estimate the level of systematic errors by considering different normalization of the extinction curve adopted by Wegner (2002). Our catalog of 782 R_V and A_V values and their errors is available in the electronic form on the World Wide Web.Comment: 20 pages, 7 figures, submitted to Acta Astronomic

Optical photometry and spectral classification in the field of the open cluster NGC 6996 in the North America Nebula

We present and discuss broad band CCD UBV(I)c photometry and low resolution spectroscopy for stars in the region of the open cluster NGC 6996, located in the North America Nebula. The new data allow us to tightly constrain the basic properties of this object. We revise the cluster size, which in the past has been significantly underestimated. The width of the Main Sequence is mainly interpreted in terms of differential reddening, and indeed the stars' color excess EB-V ranges from 0.43 to 0.65, implying the presence of a significant and evenly distributed dust component. We cross-correlate our optical photometry with near infrared photometry from 2MASS, and by means of spectral classification we are able to build extinction curves for an handful of bright members. We find that the reddening slope and the total to selective absorption ratio Rv toward NGC 6996 are anomalous. Moreover the reddening-corrected colors and magnitudes allow us to derive estimates for the cluster distance and age, which turn out to be 760 ± 70 pc (V0 - Mv = 9.4 ± 0.2) and ∼350 Myr, respectively. Based on our results, we suggest that NGC 6996 is located in front of the North America Nebula, and does not seem to have any apparent relationship with it.Facultad de Ciencias Astronómicas y Geofísica

Can CCM law properly represent all extinction curves?

We present the analysis of a large sample of lines of sight with extinction curves covering wavelength range from near-infrared (NIR) to ultraviolet (UV). We derive total to selective extinction ratios based on the Cardelli, Clayton and Mathis (1989, CCM) law, which is typically used to fit the extinction data both for diffuse and dense interstellar medium. We conclude that the CCM law is able to fit most of the extinction curves in our sample. We divide the remaining lines of sight with peculiar extinction into two groups according to two main behaviors: a) the optical/IR or/and UV wavelength region cannot be reproduced by the CCM formula; b) the optical/NIR and UV extinction data are best fit by the CCM law with different values of R_V. We present examples of such curves. The study of both types of peculiar cases can help us to learn about the physical processes that affect dust in the interstellar medium, e.g., formation of mantles on the surface of grains, evaporation, growing or shattering.Comment: 6 pages, 4 figures, in "Light, dust and chemical evolution", Journal of Physics: Conference Serie

The vertical transport of methane from different potential emission types on Mars

The contrasting evolutionary behavior of the vertical profile of methane from three potential release scenarios is analysed using a global circulation model with assimilated temperature profiles. Understanding the evolving methane distribution is essential for interpretation of future retrievals of the methane vertical profile taken by instruments on the ExoMars Trace Gas Orbiter spacecraft. We show that at methane release rates constrained by previous observations and modelling studies, discriminating whether the methane source is a sustained or instantaneous surface emission requires at least ten sols of tracking the emission. A methane source must also be observed within five to ten sols of the initial emission to distinguish whether the emission occurs directly at the surface or within the atmosphere via destabilization of metastable clathrates. Assimilation of thermal data is shown to be critical for the most accurate back-tracking of an observed methane plume to its origin

Optical photometry and spectral classification in the field of the open cluster NGC 6996 in the North America Nebula

We present and discuss broad band CCD UBV(I)c photometry and low resolution spectroscopy for stars in the region of the open cluster NGC 6996, located in the North America Nebula. The new data allow us to tightly constrain the basic properties of this object. We revise the cluster size, which in the past has been significantly underestimated. The width of the Main Sequence is mainly interpreted in terms of differential reddening, and indeed the stars' color excess EB-V ranges from 0.43 to 0.65, implying the presence of a significant and evenly distributed dust component. We cross-correlate our optical photometry with near infrared photometry from 2MASS, and by means of spectral classification we are able to build extinction curves for an handful of bright members. We find that the reddening slope and the total to selective absorption ratio Rv toward NGC 6996 are anomalous. Moreover the reddening-corrected colors and magnitudes allow us to derive estimates for the cluster distance and age, which turn out to be 760 ± 70 pc (V0 - Mv = 9.4 ± 0.2) and ∼350 Myr, respectively. Based on our results, we suggest that NGC 6996 is located in front of the North America Nebula, and does not seem to have any apparent relationship with it.Facultad de Ciencias Astronómicas y Geofísica

Optical photometry and spectral classification in the field of the open cluster NGC 6996 in the North America Nebula

We present and discuss broad band CCD $UBV(I)_C$ photometry and low resolution spectroscopy for stars in the region of the open cluster NGC 6996, located in the North America Nebula. The new data allow us to tightly constrain the basic properties of this object. We revise the cluster size, which in the past has been significantly underestimated. The width of the Main Sequence is mainly interpreted in terms of differential reddening, and indeed the stars' color excess $E_{B-V}$ ranges from 0.43 to 0.65, implying the presence of a significant and evenly distributed dust component. We cross-correlate our optical photometry with near infrared from 2MASS, and by means of spectral classification we are able to build up extinction curves for an handful of bright members. We find that the reddening slope and the total to selective absorption ratio $R_V$ toward NGC 6996 are anomalous. Moreover the reddening corrected colors and magnitudes allow us to derive estimates for the cluster distance and age, which turn out to be $760 \pm 70 pc$ ($V_{0}-M_{V} = 9.4 \pm 0.2$) and $\sim 350$ Myr, respectively. Basing on our results, we suggest that NGC 6996 is located in front of the North America Nebula, and does not seem to have any apparent relationship with it.Comment: 19 pages, 12 eps figures, in press in A&

Dust properties along anomalous extinction sightlines. II. Studying extinction curves with dust models

The large majority of extinction sight lines in our Galaxy obey a simple relation depending on one parameter, the total-to-selective extinction coefficient, Rv. Different values of Rv are able to match the whole extinction curve through different environments so characterizing normal extinction curves. In this paper more than sixty curves with large ultraviolet deviations from their best-fit one parameter curve are analyzed. These curves are fitted with dust models to shed light into the properties of the grains, the processes affecting them, and their relations with the environmental characteristics. The extinction curve models are reckoned by following recent prescriptions on grain size distributions able to describe one parameter curves for Rv values from 3.1 to 5.5. Such models, here extended down to Rv=2.0, allow us to compare the resulting properties of our deviating curves with the same as normal curves in a self-consistent framework, and thus to recover the relative trends overcoming the modeling uncertainties. Such curves represent the larger and homogeneous sample of anomalous curves studied so far with dust models. Results show that the ultraviolet deviations are driven by a larger amount of small grains than predicted for lines of sight where extinction depends on one parameter only. Moreover, the dust-to-gas ratios of anomalous curves are lower than the same values for no deviating lines of sight. Shocks and grain-grain collisions should both destroy dust grains, so reducing the amount of the dust trapped into the grains, and modify the size distribution of the dust, so increasing the small-to-large grain size ratio. Therefore, the extinction properties derived should arise along sight lines where shocks and high velocity flows perturb the physical state of the interstellar medium living their signature on the dust properties. (Abridged version)Comment: 31 pages,12 figures; accepted for publication in A&

Variability of the methane trapping in martian subsurface clathrate hydrates

Recent observations have evidenced traces of methane CH4 heterogeneously distributed in the martian atmosphere. However, because the lifetime of CH4 in the atmosphere of Mars is estimated to be around 300-600 years on the basis of photochemistry, its release from a subsurface reservoir or an active primary source of methane have been invoked in the recent literature. Among the existing scenarios, it has been proposed that clathrate hydrates located in the near subsurface of Mars could be at the origin of the small quantities of the detected CH4. Here, we accurately determine the composition of these clathrate hydrates, as a function of temperature and gas phase composition, by using a hybrid statistical thermodynamic model based on experimental data. Compared to other recent works, our model allows us to calculate the composition of clathrate hydrates formed from a more plausible composition of the martian atmosphere by considering its main compounds, i.e. carbon dioxyde, nitrogen and argon, together with methane. Besides, because there is no low temperature restriction in our model, we are able to determine the composition of clathrate hydrates formed at temperatures corresponding to the extreme ones measured in the polar caps. Our results show that methane enriched clathrate hydrates could be stable in the subsurface of Mars only if a primitive CH4-rich atmosphere has existed or if a subsurface source of CH4 has been (or is still) present.Comment: Accepted for publication in Planetary and Space Scienc

Search for hydrogen peroxide in the Martian atmosphere by the Planetary Fourier Spectrometer onboard Mars Express

We searched for hydrogen peroxide (H2O2) in the Martian atmosphere using data measured by the Planetary Fourier Spectrometer (PFS) onboard Mars Express during five martian years (MY27-31). It is well known that H2O2 plays a key role in the oxidizing capacity of the Martian atmosphere. However, only a few studies based on ground-based observations can be found in the literature. Here, we performed the first analysis of H2O2 using long-term measurements by a spacecraft-borne instrument. We used the ν4 band of H2O2 in the spectral range between 359 cm-1 and 382 cm-1 where strong features of H2O2 are present around 362 cm-1 and 379 cm-1. Since the features were expected to be very weak even at the strong band, sensitive data calibrations were performed and a large number of spectra were selected and averaged. We made three averaged spectra for different seasons over relatively low latitudes (50°S-50°N). We found features of H2O2 at 379 cm-1, whereas no clear features were detected at 362 cm-1 due to large amounts of uncertainty in the data. The derived mixing ratios of H2O2 were close to the detection limits: 16 ± 19 ppb at Ls = 0-120°, 35 ± 32 ppb at Ls = 120-240°, and 41 ± 28 ppb at Ls = 240-360°. The retrieved value showed the detection of H2O2 only for the third seasonal period, and the values in the other periods provided the upper limits. These long-term averaged abundances derived by the PFS generally agreed with the ones reported by ground-based observations. From our derived mixing ratio of H2O2, the lifetime of CH4 in the Martian atmosphere is estimated to be several decades in the shortest case. Our results and sporadic detections of CH4 suggest the presence of strong CH4 sinks not subject to atmospheric oxidation. <P /