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1.
R. Poggiani 《New Astronomy》2010,15(8):657-661
We report the spectroscopic follow-up of the nova V5558 Sgr carried out in 2008 and 2009 at the Loiano Observatory, Italy, during the decline stage, as a part of an ongoing monitoring campaign. The spectra suggest that V5558 Sgr has entered the nebular stage, as shown by the presence of high ionization lines, and confirm that it is very similar to the slow nova V723 Cas, as previously suggested. We have estimated the decline time by three magnitudes, 170 ± 2 days, typical of slow novae. We have revised the estimation of the absolute magnitude at maximum (?6.3 to ?5.9) and the distance (1.3–1.6 kpc) of V5558 Sgr, in agreement with previous results. The revised white dwarf mass (0.58–0.63 M⊙) confirms that V5558 Sgr is a critical system whose mass is close to the lower limit to trigger the nova outburst.  相似文献   

2.
We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of V1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s−1. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.  相似文献   

3.
Orbital period changes of the eclipsing binaries GO Cyg and GW Cep are explained by the light-time effect for the first time. New minima of the eclipsing binary AR Aur improve the predicted light-time orbit. The light-time orbit with the quadratic ephemeris of the binary matches the new observations of V505 Sgr better than the linear one. As the light-time effect fits in corresponding O − C diagrams of all four systems have been reaching extreme values, the observations of minima times in forthcoming years are highly desirable.  相似文献   

4.
This paper is the next one in the series of our works aimed at determining the atmospheric parameters and chemical composition of southern-hemisphere Cepheids. We present the results of our study for six bright Cepheids: V Cen, V737 Cen, BB Sgr, W Sgr, X Sgr, and Y Sgr. We have analyzed 14 high-resolution spectra taken with the 1.9-m telescope at the South African Astronomical Observatory. In addition to determining the chemical composition and atmospheric parameters, we point out and discuss several features in the spectra of individual Cepheids. In particular,we have detected emission in the cores of the Hβ and Hα lines forWSgr near its maximum light, while X Sgr shows the splitting of metal absorption lines into individual components without any change of the hydrogen lines. These peculiarities can be explained by different manifestations of shock waves in the Cepheid atmospheres and by the presence of circumstellar envelopes around X Sgr and W Sgr. The chemical composition has been estimated for V737 Cen, BB Sgr, and X Sgr for the first time. On the whole, our abundance estimates for α-elements, iron-peak elements, and r- and s-process elements are close to the solar ones for all objects, carbon is underabundant, the oxygen abundance is nearly solar, the “odd” elements, Na and Al, are overabundant (except X Sgr), magnesium is underabundant for V Cen and X Sgr and overabundant for the remaining objects. Such a chemical composition is typical of yellow supergiants after the first dredge-up. Keywords: Cepheids, spectra, atmospheric parameters, chemical composition.  相似文献   

5.
The optical photometric evolution of the final helium flash object V4334 Sgr from 1994 to 2001 is described. The rise to optical maximum (1994–1996) is characterized by a continuous increase of colour indices, indicating a slowly expanding, cooling pseudo-photosphere. This photosphere became stationary in 1997. In the following years, the object underwent brightnessdeclines of increasing strength, which are similar in character to the `reddeclines' of RCB stars. The fading of V4334 Sgr is more dramatic than anybrightness decline of an RCB star: at present, only 10-5 of the visual light reaches the observer. Most short-wavelength radiation is absorbed by a dust envelopethat completely surrounds the star, and is re-radiated in the infrared. The sparse optical data of 2000–2001 show that the obscuration has not increased in strength any more. The light curve of V4334 Sgr is similar to that of the final helium flash object V605 Aql which erupted in 1919.  相似文献   

6.
Analysis of the anisotropy of the -ray background observed by Clarket al. (1968) has lead to the suggestion that part of the anisotropy be due to a source at the Galactic Centre. The complex structure of radio and infrared emission of the region of Sgr A is considered. The -ray flux arising from Compton scattering and 0 decay is calculated. For a value of the magnetic field H10–4 Gauss, corresponding to equipartition, the -ray flux above 100 MeV at Earth from the Sgr A source is expected to lie between 10–6 and 10–5 gamma/cm2 sec. Such a source should be detectable in the near future.  相似文献   

7.
Procedures and results of computations of model atmospheres andspectral energy distributions of R CrB, V4334 Sgr and relatedobjects are discussed. The sequence of SEDs computed for a gridof model atmospheres with 4000 < Teff < 7000, 0 < logg < 1 provides a unique possibility to analyze the temporalchanges of physical parameters of V 4334 Sgr and related objectsin the framework of a self-consistent approach.  相似文献   

8.
The new spectroscopic observation of MV Sgr obtained at ESO in 1987 July shows enhanced emission lines of He I λ3889, [SII] λ4068 relative to the observations discussed by Jeffreyet al. (1988). The presence of [SII] λ4068 indicates the presence of planetary-nebulae-like envelope around the star. Although the radial velocity of the absorption lines and Fe II emission lines do agree with the velocity given by Jeffreyet al., the [SII] λ4068 and probably He I emission lines appear to behave differently. Based on observations collected at the European Southern Observatory, La Silla, Chile.  相似文献   

9.
We report mapping observations of a 35 pc × 35 pc region covering the Sgr B2 molecular cloud complex in the 13CO (3-2) and the CS (7-6) lines using the ASTE 10 m telescope with high angular resolution. The central region was mapped also in the C18O (3-2) line. The images not only reproduce the characteristic structures noted in the preceding millimeter observations, but also highlight the interface of the molecular clouds with a large velocity jump of a few tens of km s−1. These new results further support the scenario that a cloud–cloud collision has triggered the formation of massive cloud cores, which form massive stars of Sgr B2. Prospects of exciting science enabled by ALMA are discussed in relation to these observations.  相似文献   

10.
VLA observations at 6 cm have been obtained for three hydrogen-deficient objects υ Sgr, V 348 Sgr, and A bell 58. A bell 58 was also observed at 2 cm. Only upper limits to the flux density could be set for these sources. A new radio source at 6 cm was found in the field of υ Sgr. The upper limit for 6 cm flux density of V348 Sgr sets an upper limit to its reddening asE(B–V) ≤ 0.65. The hydrogen deficient planetary nebula A 58 shows much lower radio flux than expected from the infrared-radio flux density relationship of planetary nebulae. National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National Science Foundation, USA.  相似文献   

11.
The recent detection of TeV gamma-radiation from the direction of the Galactic Center within several arc-minutes around Sgr A* is the first model-independent evidence of existence of high-energy particle accelerator(s) in the central 10 pc region of our Galaxy. This is an extraordinary site that harbours many remarkable objects with the compact radio source Sgr A* – a hypothetical super-massive black hole (SMBH) – in the dynamical center of the Galaxy. Here we explore the possible direct and indirect links of the reported TeV emission to the SMBH. We show that at least three γ-ray production scenarios that take place close to the event horizon of the SMBH can explain the reported TeV fluxes. An alternative (or additional) channel of TeV radiation is related to the run-away protons accelerated in Sgr A*. Quasi-continuous injection of relativistic protons into the surrounding dense gas environment initiates detectable high-energy gamma-ray emission. The absolute flux and the energy spectrum of this radiation component strongly depend on the history of particle injection and the character of diffusion of protons during the last 105 yr. For a reasonable combination of a few model parameters, one can explain the detected gamma-ray flux solely by this diffuse component.  相似文献   

12.
We present multi-colour CCD observations of the low-temperature contact binaries, V453 Mon and V523 Cas. Their light curves are modelled to determine a new set of stellar and orbital parameters. Analysis of mid-eclipse times yields a new linear ephemeris for both systems. A period decrease (dP/dt=2.3×10−7 days/yr) in V453 Mon is discovered. V523 Cas, however, is detected to show a period increase (dP/dt=9.8×10−8 days/yr) because of the mass transfer of a rate of 1.1×10−7 M⊙ yr−1, from a less massive donor. Using these findings we can determine the physical parameters of the components of V523 Cas to be M 1=0.76 (3)M ⊙, M 2=0.39 (2)M ⊙, R 1=0.74 (2)R ⊙, R 2=0.55 (2)R ⊙, L 1=0.19 (3)L ⊙, L 2=0.14 (3)L ⊙, and the distance of system as 46(9) pc.  相似文献   

13.
The central star V4334 Sgr (Sakurai's Object) of the planetary nebula PN G010.4+04.4 underwent in 1995–1996 the rare event of a very late helium flash. It represents only one out of two such events during the era of modern astronomy (the other event was V605 Aql = Nova Aql 1919). All the other prominent objects of that type originate from events occurring several thousands of years ago (e.g. A30, A78). Thus it is of special interest for stellar evolution theory to model the detailed observations obtained during the last four years. Those models depend essentially on basic stellar parameters like effective temperature, surface gravity and stellar radius. Most of them depend strongly on the assumed distance to the object. Some models may give some constraints on this parameter, but most of them depend on the assumption as input parameter. Hence to determine a reliable distance is of considerable significance. This should be obtained through models that give us lower and upper boundaries, or through means which are independent of models. The detailed review, by using every kind of determination available up to now, leads to a Galactic foreground extinction of E B–V =0m75 ±0.05 and a distance of D = 2.0-0.6 +1.0 kpc.  相似文献   

14.
We report the spectroscopic observations of nova V2670 Oph secured at the Loiano Observatory, Italy, 39 days after the outburst. The spectra of V2670 Oph showed auroral lines, suggesting that ionization was increasing. V2670 Oph is a fast nova, with a decline rate by three magnitudes of 42 days. We have estimated the absolute magnitude at maximum and the distance of V2670 Oph. The nova achieved an absolute magnitude at maximum in the interval −7.9,…,−7.4. The distance of V2670 Oph is in the range 4.7–5.8 kpc.  相似文献   

15.
An experiment was conducted in conjunction with the total solar eclipse on 29 March 2006 in Libya to measure both the electron temperature and its flow speed simultaneously at multiple locations in the low solar corona by measuring the visible K-coronal spectrum. Coronal model spectra incorporating the effects of electron temperature and its flow speed were matched with the measured K-coronal spectra to interpret the observations. Results show electron temperatures of (1.10±0.05) MK, (0.70±0.08) MK, and (0.98±0.12) MK, at 1.1 R ⊙ from Sun center in the solar north, east and west, respectively, and (0.93±0.12) MK, at 1.2 R ⊙ from Sun center in the solar west. The corresponding outflow speeds obtained from the spectral fit are (103±92) km s−1, (0+10) km s−1, (0+10) km s−1, and (0+10) km s−1. Since the observations were taken only at 1.1 R ⊙ and 1.2 R ⊙ from Sun center, these speeds, consistent with zero outflow, are in agreement with expectations and provide additional confirmation that the spectral fitting method is working. The electron temperature at 1.1 R ⊙ from Sun center is larger at the north (polar region) than the east and west (equatorial region).  相似文献   

16.
A detailed analysis of characteristics of coronal mass ejections and flares associated with deca-hectometer wavelength type-II radio bursts (DH-CMEs and DH-flares) observed in the period 1997–2008 is presented. A sample of 62 limb events is divided into two populations known as after-flare CMEs (AF-CMEs) and before-flare CMEs (BF-CMEs) based on the relative timing of the flare and CME onsets. On average, AF-CMEs (1589 km s−1) have more speed than the BF-CMEs (1226 km s−1) and the difference between mean values are highly significant (P∼2%). The average CME nose height at the time of type-II start is at larger distance for AF-CMEs than the BF-CMEs (4.89 and 3.84 R o, respectively). We found a good anti-correlation for accelerating (R a=−0.89) and decelerating (R d=−0.78) AF-CMEs. In the case of decelerating BF-CMEs, the correlation seems to be similar to that for decelerating AF-CMEs (R d=−0.83). The number of decelerating AF-CMEs is 51% only; where as, the number of decelerating BF-CMEs is 83%. The flares associated with BF-CMEs have shorter rise and decay times than flares related to AF-CMEs. We found statistically significant differences between the two sets of associated DH-type-II bursts characteristics: starting frequency (P∼4%), drift rate (P∼1%), and ending frequency (P∼6%). The delay time analysis of DH-type-II start and flare onset times shows that the time lags are longer in AF-CME events than in BF-CME events (P≪1%). From the above results, the AF-CMEs which are associated with DH-type-II bursts are found to be more energetic, associated with long duration flares and DH-type-IIs of lower ending frequencies.  相似文献   

17.
The GALEX General Data Release 4/5 includes 174 spectroscopic tiles, obtained from slitless grism observations, for a total of more than 60 000 ultraviolet spectra. We have determined statistical properties of the sample of GALEX stars. We have defined a suitable system of spectroscopic indices, which measure the main mid-UV features at the GALEX low spectral resolution and we have employed it to determine the atmospheric parameters of stars in the range 4500≲T eff≲9000 K. Our preliminary results indicate that the majority of the sample is formed by main sequence F- and G-type stars, with metallicity [M/H]≳−1 dex.  相似文献   

18.
A dm-radio emission with fiber bursts observed on 11 July 2005 was analyzed using wavelet filtration and spectral methods. In filtered radio spectra we found structures with different characteristic period P and frequency drift FD: i) fiber substructures (composed of dot emissions) with P 1≈ 0.5 s, FD1=− 87 MHz s−1 on average, ii) fiber structures with P 2≈1.9 s, and iii) drifting structures with P 3≈81.4 s, FD2=− 8.7, + 98.5, and − 21.8 MHz s−1. In the wavelet spectra we recognized patterns having the form of tadpoles. They were detected with the same characteristic periods P as found for the filtered structures. The frequency drift of the tadpole heads is found to be equal to the frequency drift of some groups of fibers for the long-period wavelet tadpoles (P 3) and to the frequency drift of individual fibers for the short-period tadpoles (P 2). Considering these wavelet tadpoles as signatures of propagating magnetoacoustic wave trains, the results indicate the presence of several wave trains in the fibers’ source. While the long-period wave trains trigger or modulate a whole group of fibers, the short-period ones look like being connected with individual fiber bursts. This result supports the model of fibers based on magnetoacoustic waves. Using a density model of the solar atmosphere we derived the velocities of the magnetoacoustic waves, 107 and 562 km s−1, and setting them equal to the Alfvén ones we estimated the magnetic field in the source of fiber bursts as 10.7 and 47.8 G.  相似文献   

19.
We carried out optical and Hα photometry of two contact binaries (V861 Herculis, EQ Tauri). The light curve modeling revealed stellar spots in both contact systems and strong Hα excess in the position of the observed stellar spots. A correlation was found between the V−R and R−Hα colour indices of V861 Her.  相似文献   

20.
We present CCD BV and JHK s 2MASS photometric data for the open cluster NGC 1513. We observed 609 stars in the direction of the cluster up to a limiting magnitude of V∼19 mag. The star-count method showed that the centre of the cluster lies at α 2000=04 h 09 m 36 s , δ 2000=49°28′43″ and its angular size is r=10 arcmin. The optical and near-infrared two-colour diagrams revealed the colour excesses in the direction of the cluster as E(B−V)=0.68±0.06, E(J−H)=0.21±0.02 and E(J−K s )=0.33±0.04 mag. These results are consistent with normal interstellar extinction values. Optical and near-infrared Zero Age Main-Sequences (ZAMS) provided an average distance modulus of (m−M)0=10.80±0.13 mag, which can be translated into a distance of 1440±80 pc. Finally, using Padova isochrones we determined the metallicity and age of the cluster as Z=0.015±0.004 ([M/H]=−0.10±0.10 dex) and log (t/yr)=8.40±0.04, respectively.  相似文献   

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