ABSTRACT
In this paper, we report on spatial intensity interferometry measurements within the Hα line on two stars: the Luminous Blue Variable supergiant P Cygni and the late-type B supergiant ...Rigel. The experimental setup was upgraded to allow simultaneous measurement of two polarization channels, instead of one in our previous setup, and the zero baseline correlation function on-sky to validate independent estimates obtained from the stellar spectrum and the instrumental spectral throughput. Combined with simultaneous spectra measurements and based on radiative transfer models calculated with the code CMFGEN, we were able to fit our measured visibility curves to extract the stellar distances. Our distance determinations for both P Cygni (1.61 ± 0.18 kpc) and Rigel (0.26 ± 0.02 kpc) agree very well with the values provided by astrometry with the Gaia and Hipparcos missions, respectively. This result for Rigel was obtained by adopting a stellar luminosity of L⋆ = 123 000 L⊙, which is reported in the literature as being consistent with the Hipparcos distance to Rigel. However, due to the lack of consensus on Rigel’s luminosity, we also explore how the adoption of the stellar luminosity in our models affects our distance determination for Rigel. In conclusion, we support, in an independent way, the distance to Rigel as the one provided by the Hipparcos mission, when taking the luminosity of 123 000 L⊙ at face value. This study is the first successful step towards extending the application of the Wind Momentum Luminosity Relation method for distance calibration from an LBV supergiant to a more normal late-type B supergiant.
ABSTRACT
We present intensity interferometry of the luminous blue variable P Cyg in the light of its H α emission performed with 1 m-class telescopes. We compare the measured visibility points to ...synthesized interferometric data based on the CMFGEN physical modelling of a high-resolution spectrum of P Cyg recorded almost simultaneously with our interferometry data. Tuning the stellar parameters of P Cyg and its H α linear diameter, we estimate the distance of P Cyg as 1.56 ± 0.25 kpc, which is compatible within 1σ with 1.36 ± 0.24 kpc reported by the Gaia DR2 catalogue of parallaxes recently published. Both the values are significantly smaller than the canonic value of 1.80 ± 0.10 kpc usually adopted in literature. Our method used to calibrate the distance of P Cyg can apply to very massive and luminous stars both in our Galaxy and neighbouring galaxies, and can improve the so-called wind momentum–luminosity relation that potentially applies to calibrate cosmological candles in the local Universe.
O presente artigo objetiva discutir acerca dos relevos residuais do semiárido nordestino, enfocando um estudo de caso realizado no município de Alexandria no Alto Oeste Potiguar, Rio Grande do Norte, ...através do qual apresenta-se como heranças das feições geomorfológicas marcantes da paisagem semiárida do sertão do oeste potiguar nordestino. No tocante os procedimentos metodológicos, a pesquisa é sistematizada em duas etapas: gabinete – levantamento bibliográfico e geocartográfico – e campo – visita e reconhecimento da área em estudo. Consideramos assim os estudos teóricos de Ab’Saber (2003), Bertrand (2004), Lima et. al. (2009), Maia e Nascimento (2015; 2018), Souza e Oliveira (2002), além de interpretações cartográficas acerca das características geológico-geomorfológicas da área com base na CPRM (2005). Neste trabalho, verificamos a presença de relevos residuais predominantes, bem como extensas áreas de lajedos, que conferem uma paisagem exuberante no interior do sertão potiguar nordestino, de uma morfologia granítica, resultante do processo formativo de processos endógenos e exógenos, como resposta à constituição da paisagem como herança, sobretudo os inselbergs.
O Semiárido brasileiro é um espaço dotado de singularidades naturais, potencialidades e limitações, que em sua essência resguarda paisagens de belezas cênicas com alto potencial à Geodiversidade. É ...nesse contexto que o trabalho tem por objetivo analisar como os geoparques, por meio da Geodiversidade, podem fomentar o desenvolvimento territorial do Semiárido brasileiro. Para tanto, a pesquisa foi sistematizada em três etapas principais: levantamento bibliográfico, revisão sistemática integrativa da literatura sobre Geodiversidade e Geodiversidade AND Semiárido brasileiro, e coleta de dados secundários. O conceito de Geodiversidade é relativamente novo e integra uma série de outros temas, estando intimamente ligado aos elementos abióticos do espaço terrestre, sobretudo de expressão geológica e geomorfológica. Por sua vez, os Geoparques, como unidades territoriais demarcadas pela presença de patrimônios geológico-geomorfológicos únicos de alto valor em sua diversidade e aplicações, podem possibilitar o desenvolvimento territorial dos locais que abrigam essas singularidades. Nesse sentido, entende-se que a Geodiversidade por meio dos Geoparques pode fomentar o desenvolvimento territorial do Semiárido brasileiro, na medida em que possibilita uma série de atividades satélites, como o Geoturismo.
Context. Among intermediate-mass and massive stars, Be stars are the fastest rotators in the main sequence (MS) and, as such, these stars are a cornerstone to validate models of structure and ...evolution of rotating stars. Several phenomena, however, induce under- or overestimations either of their apparent V sini, or true velocity V. Aims. In the present contribution we aim at obtaining distributions of true rotational velocities corrected for systematic effects induced by the rapid rotation itself, macroturbulent velocities, and binarity. Methods. We study a set of 233 Be stars by assuming they have inclination angles distributed at random. We critically discuss the methods of Cranmer and Lucy-Richardson, which enable us to transform a distribution of projected velocities into another distribution of true rotational velocities, where the gravitational darkening effect on the V sini parameter is considered in different ways. We conclude that iterative algorithm by Lucy-Richardson responds at best to the purposes of the present work, but it requires a thorough determination of the stellar fundamental parameters. Results. We conclude that once the mode of ratios of the true velocities of Be stars attains the value V/V sub(c)Asymp totically = to 0.77 in the main-sequence (MS) evolutionary phase, it remains unchanged up to the end of the MS lifespan. The statistical corrections found on the distribution of ratios V/V sub(c) for overestimations of V sini, due to macroturbulent motions and binarity, produce a shift of this distribution toward lower values of V/V sub(c) when Be stars in all MS evolutionary stages are considered together. The mode of the final distribution obtained is at V/V sub(c)Asymp totically = to 0.65. This distribution has a nearly symmetric distribution and shows that the Be phenomenon is characterized by a wide range of true velocity ratios 0.3 <, ~ V/V sub(c)<, ~ 0.95. It thus suggests that the probability that Be stars are critical rotators is extremely low. Conclusions. The corrections attempted in the present work represent an initial step to infer indications about the nature of the Be-star surface rotation that will be studied in the second paper of this series.
A compreensão em torno do semiárido brasileiro se deu pela transição de paradigmas de combate à seca e convivência com o semiárido, incorporados nas intervenções estatais. Assim, o escrito objetiva ...identificar as políticas públicas no contexto do paradigma da convivência com o semiárido na sub-bacia hidrográfica do Rio Figueiredo, no Ceará. Para tanto, realizou-se revisão na literatura sobre o semiárido brasileiro, combate à seca e convivência com o semiárido; foram levantados dados da infraestrutura hídrica na área de estudo. Desse modo, por essa infraestrutura ter se dado como política pública histórica, refletem nos municípios da sub-bacia sob a forma de açudes de médio e grande porte e tecnologias sociais. Estas se deram sob os dois paradigmas, cada uma a sua maneira, representam importante empreendimento de estoque de água em ambiente marcado pela irregularidade espaço-temporal das chuvas e intermitência dos rios, constituindo-se um dos pilares da convivência com o semiárido.
Context.
The infrared (IR) excess of classical Cepheids is seldom studied and poorly understood despite observational evidence and the potential for its contribution to induce systematics on the ...period-luminosity (PL) relation used in the calibration of the extragalactic distance scale.
Aims.
This study aims to understand the physical origin of the IR excess found in the spectral energy distribution (SED) of 5 Cepheids: RS Pup (
P
= 41.46d),
ζ
Gem (
P
= 10.15d),
η
Aql (
P
= 7.18d), V Cen (
P
= 5.49d) and SU Cyg (
P
= 3.85d).
Methods.
A time series of atmospheric models along the pulsation cycle were fitted to a compilation of data, including optical and near-IR photometry,
Spitzer
spectra (secured at a specific phase), interferometric angular diameters, effective temperature estimates, and radial velocity measurements.
Herschel
images in two bands were also analyzed qualitatively. In this fitting process, based on the SPIPS algorithm, a residual was found in the SED, whatever the pulsation phase, and for wavelengths larger than about 1.2
μ
m, which corresponds to the so-determined infrared excess of Cepheids. This IR excess was then corrected from interstellar medium absorption in order to infer the presence (or absence) of dust shells and was, ultimately, used in order to fit a model for a shell of ionized gas.
Results.
For all Cepheids, we find a continuum IR excess increasing up to approximately −0.1 magnitudes at 30
μ
m, which cannot be explained by a hot or cold dust model of CircumStellar Environment (CSE). However, a weak but significant dust emission at 9.7
μ
m is found for
ζ
Gem,
η
Aql and RS Pup, while clear interstellar clouds are seen in the
Herschel
images for V Cen and RS Pup. We show, for the first time, that the IR excess of Cepheids can be explained by free–free emission from a thin shell of ionized gas, with a thickness of ≃15% of the star radius, a mass of 10
−9
−10
−7
M
⊙
and a temperature ranging between 3500 and 4500 K.
Conclusions.
The presence of a thin shell of ionized gas around Cepheids must be tested with interferometers operating in the visible or mid-IR, or using radio telescopes. The impact of such CSEs of ionized gas on the PL relation of Cepheids also calls for further investigation.
Future large arrays of telescopes, used as intensity interferometers, can be used to image the surfaces of stars with unprecedented angular resolution. Fast-rotating, hot stars are particularly ...attractive targets for intensity interferometry since shorter (blue) wavelength observations do not pose additional challenges. Starting from realistic surface brightness simulations of fast-rotating stars, we discuss the capabilities of future intensity interferometers for imaging effects such as gravity darkening and rotational deformation. We find that two-telescope intensity correlation data allow reasonably good imaging of these phenomena, but can be improved with additional higher order (e.g. three-telescope) correlation data, which contain some Fourier phase information.
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