P. Nedialkov - Academia.edu (original) (raw)

Papers by P. Nedialkov

Research in Astronomy and Astrophysics, 2021

We study Luminous Blue Variable (LBV) candidate J004341.84+411112.0 in the Andromeda galaxy. We p... more We study Luminous Blue Variable (LBV) candidate J004341.84+411112.0 in the Andromeda galaxy. We present optical spectra of the object obtained with the 6 m telescope of BTA SAO RAS. The candidate shows typical LBV features in its spectra: broad and strong hydrogen lines and the He i lines with P Cygni profiles. Its remarkable spectral resemblance to the well known LBV P Cygni suggests a common nature of the objects and supports LBV classification of J004341.84+411112.0. We estimate the temperature, reddening, radius and luminosity of the star using its spectral energy distribution. Obtained bolometric luminosity of the candidate (M bol = −10.41 ± 0.12 mag) is quite similar to those of known LBV stars in the Andromeda galaxy. We analyzed a ten year light curve of the object in R filter. The candidate demonstrates photometric variations of the order of 0.4 mag, with an overall brightness increasing trend ΔR > 0.1 mag. Therewith, the corresponding color variations of the object are ...

Abstract. Based on 2MASS J and Ks photometry for the open star clusters NGC 2383, NGC 2384, Pismi... more Abstract. Based on 2MASS J and Ks photometry for the open star clusters NGC 2383, NGC 2384, Pismis 6, Pismis 8 and using color magnitude diagrams with isochrones fit, we found an age of log(age) = 8.3 (200±6Myr) for NGC 2383 and log(age) = 6.9 (8±6Myr) for NGC 2384. For Pismis 6 and Pismis 8 we adopted a range of log(age) = 6- 7 (1- 10 Myr). Because they similar ages, Pismis 6 and Pismis 8 may have been formed in the same Giant Molecular Cloud, and we concluded they are a good candidate for a binary system. In the case of NGC 2383 and NGC 2384, because the big age difference found we conclude that most probably they are born in different environments and as well are not physically connected. PACS number: 98.20.Di 1

GROUND-BASED ASTRONOMY IN RUSSIA. 21ST CENTURY. Proceedings of the All-Russian Conference, 2020

Calibrated Tully-Fisher relations ''absolute magnitude - HI line width'' for spir... more Calibrated Tully-Fisher relations ''absolute magnitude - HI line width'' for spiral galaxies oriented edge-on are derived from published data. The slope of the linear fits, especially in R- and I- bands, is close to the value of -10, as predicted by the theory. The standard errors of the relations in B, V, R and I bands are 0.40, 0.36, 0.31 and 0.34 mag, corresponding to relative distance errors of 18%, 17%, 14% and 16%, respectively. The magnitude of an edge-on galaxy appears to be a good distance indicator.

Monthly Notices of the Royal Astronomical Society, 2021

We report here the first results from a 15-yr long variability monitoring of the z = 2.0 quasar Q... more We report here the first results from a 15-yr long variability monitoring of the z = 2.0 quasar QSO B1312+7837. It shows luminosity changes with a period P ∼ 6.13 yr (P ∼ 2.04 yr at rest frame) and amplitude of ∼0.2 mag, superimposed on a gradual dimming at a rate of ∼0.55 mag per 100 yr. Two false periods associated with power peaks in the data windowing function were discarded. The measured period is confirmed with a bootstrapping Monte Carlo simulation. A damped random walk model yields a better fit to the data than a sine-function model, but at the cost of employing some high-frequency variations which are typically not seen in quasars. We consider the possible mechanisms driving this variability, and conclude that orbital motion of two supermassive black holes – result from a recent galaxy merger – is a possible explanation.

Astronomy & Astrophysics, 2015

Context. M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015-the first even... more Context. M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015-the first event of this kind observed in M 31 since 1988. Very few similar events have been confirmed as of 2015. Most of them are considered to be products of stellar mergers. Aims. Results of an extensive optical monitoring of the transient in the period January-March 2015 are presented. Methods. Eight optical telescopes were used for imaging. Spectra were obtained on the Large Altazimuth Telescope (BTA), the Gran Telecsopio Canarias (GTC) and the Rozhen 2 m telescope. Results. We present a highly accurate 70 d light curve and astrometry with a 0.05 uncertainty. The colour indices reached a minimum of 2−3 d before peak brightness and rapidly increased afterwards. The spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the next 30 d. The luminosity of the transient was estimated to be 8.7 +3.3 −2.2 × 10 5 L during the optical maximum. Conclusions. Both the photometric and the spectroscopic results confirm that the object is a red nova, similar to V838 Monocerotis.

We report the discovery of a probable nova located at RA(2000) = 00h43m52.94s, Dec(2000) = +41d03... more We report the discovery of a probable nova located at RA(2000) = 00h43m52.94s, Dec(2000) = +41d03'36", which is 12' 54" east and 12' 33" south of the center of M31. Available R-band magnitudes from 5x300sec and 3x300sec co-added CCD images taken with the 50/70cm Schmidt telescope (+ SBIG STL-11000M CCD camera) at NAO Rozhen, Bulgaria: November 6.809 UT, 19.7+/-0.2; 8.853

Several properties of the M31 disk, namely: opacity, extinction law and gas-to-dust ratio are stu... more Several properties of the M31 disk, namely: opacity, extinction law and gas-to-dust ratio are studied by means of optical and near-infrared photometry of ten globular clusters and galaxies seen through the disk. The individual extinctions of these objects were estimated with respect to several sets of theoretical spectral energy distributions for globulars and galaxies. Seven targets are consistent with reddened globulars, two - with starburst galaxies and one - with an elliptical. The extinction estimates agree with semi-transparent disk (Av ~ 1) in the inter-arm regions. The total-to-selective extinction ratio in those regions 2.75 is lower on average than the typical Galactic value of Rv=3.1. We also obtained a gas-to-dust ratio, similar to that in the the Milky way. It shows no correlation with the distance from the M31 center.

In this paper we examine the gas-to-dust ratio in M31 galaxy combining radio emission data on neu... more In this paper we examine the gas-to-dust ratio in M31 galaxy combining radio emission data on neutral hydrogen (HI) and carbon oxide (CO) with the recent extinction estimates of young stellar groups (OB associations and open clusters). The fraction of sightlines where the observed N(HI)/2EB-V ratio is outside the typical Galactic range is only 1/10; however, the correlation between these two quantities remains weak. A radial dependence of this ratio, twice flatter than previous estimates, indicates absence of a strong metallicity gradient. Detection of objects behind the disk of M31 allow pencil beams estimates of the total extinction. The results confirm the plausibility of a disk model with exponentially decreasing optical thickness in radial direction. Nearly constant gas−to−dust ratio in the Galactic interstellar medium in the few kpc vicinity of the Sun [1] shows that dust and gas are well mixed and the grain properties do not vary essentially. One comes to this conclusion taki...

Monthly Notices of the Royal Astronomical Society

We study five luminous blue variable (LBV) candidates in the Andromeda galaxy and one more (MN112... more We study five luminous blue variable (LBV) candidates in the Andromeda galaxy and one more (MN112) in the Milky Way. We obtain the same-epoch near-infrared (NIR) and optical spectra on the 3.5-m telescope at the Apache Point Observatory and on the 6-m telescope of the SAO RAS. The candidates show typical LBV features in their spectra: broad and strong hydrogen lines, He i, Fe ii, and [Fe ii] lines. We estimate the temperatures, reddening, radii and luminosities of the stars using their spectral energy distributions. Bolometric luminosities of the candidates are similar to those of known LBV stars in the Andromeda galaxy. One candidate, J004341.84+411112.0, demonstrates photometric variability (about 0.27 mag in the V band), which allows us to classify it as an LBV. The star J004415.04+420156.2 shows characteristics typical of B[e] supergiants. The star J004411.36+413257.2 is classified as a Fe ii star. We confirm that the stars J004621.08+421308.2 and J004507.65+413740.8 are warm hy...

Proceedings of the International Astronomical Union

We have compiled historical observations, spanning ∼100 years, for a dozen of the best-studied LB... more We have compiled historical observations, spanning ∼100 years, for a dozen of the best-studied LBVs in the Local Group. We described how we prepared structure functions for their light-curves and calculated two parameters (the structure function slope and the characteristic time-scale) to describe the behaviour of the LBVs. The sensitivity of those parameters to the variability behaviour of the stars was tested with a number of photometric data sets. The slope of the structure function may anticorrelate with the time-scale. These types of variable stellar objects are crucial to studies of stellar variability and the final stages of stellar evolution.

We derived individual extinction values for selected red supergiant (RSGs) candidates in M31 with... more We derived individual extinction values for selected red supergiant (RSGs) candidates in M31 with broadband photometry. Taking into account their position on the colour-magnitude diagram and using a probability method, the metallicity of each star was estimated. In the range 2-15 kpc the ratio [O/H] is nearly constant. The derived individual extinctions and pencil beams values from three different gas maps (Westerbork HI, VLA HI and IRAM CO (1→0)-line survey) were used to obtain gas-to-dust ratio in M31. For the 9 most luminous stars both the ratios N(HI)/2 EB-V and N(HI+H2)/2EB-V are not very different from those in the Milky Way. Significant fractions outside the expected range of Galactic atomic gas-to-dust ratio are obtained for ~1/3 of the sample using Westerbork and for ~1/2 of the sample using VLA maps. The ratios are overestimated for objects located high above the midplane of M31 and underestimated - due to resolution effects, - for several RSGs coinciding with small HI clouds.

We present a long-term optical search for novae in M31 galaxy, based on observations taken at the... more We present a long-term optical search for novae in M31 galaxy, based on observations taken at the 2m RCC telescope and the 50/70cm Schmidt telescope at NAO Rozhen, Bulgaria. Our monitoring of the M31 central region yields ˜ 15 per cent of all newly discovered novae during the last 5 years. Here we report coordinates and R-band magnitudes (BV-band if available) for 14 newly discovered nova candidates along with a Mira variable, mistaken for nova. Finding charts and light curves are presented. Times of decline (t_{2}) are estimated and Maximum Magnitude - Rate of Decline relationship (MMRD) is constructed.

An average value of the total-to-selective-extinction ratio R_V=3.8 +/- 0.4 in M31 is obtained by... more An average value of the total-to-selective-extinction ratio R_V=3.8 +/- 0.4 in M31 is obtained by means of two independent methods and by use of the analytical formula of Cardelli, Clayton & Mathis (1989). This result differs from previous determinations as well from the `standard' value 3.1 for the Milky Way. The derived individual extinctions for blue and red luminous stars from the catalogue of Magnier et al. (1992) are in good agreement with recent estimates for several OB associations in M31 and thus the issue about the assumed optical opacity of the spiral disk still remains open. The presented list of 113 red supergiant candidates in M31 with their extinctions and luminosities contains 60 new objects of this type which are not identified in other publications. It is supplemented with further 290 stars dereddened on the base of results for their closest neighbors. The luminosity function of all red supergiant candidates and the percentage of those with progenitors over 20 ...

Compact HII regions surrounding luminous early-type stars could be recognized by their contributi... more Compact HII regions surrounding luminous early-type stars could be recognized by their contribution to the measured magnitude Rc. Applying a special dereddening procedure to a sample with BVRcIc photometry from the catalog of Magnier et al. (1992), we selected starlike objects in M31 that are excessively bright in Rc due to Hα+[S II] emission. Our results are compared with the well calibrated flux densities of compact HII regions (Walterbos & Braun 1992). The typical luminosities (~ 1037 erg/s) and the estimated absolute average magnitudes of the embedded ionizing source (MV = -7.5m) point to a very massive single star or to a small cluster of several massive stars. We also derived the nominal internal extinction of 15 blue objects with reliable individual values of RV. The intrinsic extinction within HII regions is, typically, small indicating that the dust has been swept away by stellar winds.

Research in Astronomy and Astrophysics, 2021

We study Luminous Blue Variable (LBV) candidate J004341.84+411112.0 in the Andromeda galaxy. We p... more We study Luminous Blue Variable (LBV) candidate J004341.84+411112.0 in the Andromeda galaxy. We present optical spectra of the object obtained with the 6 m telescope of BTA SAO RAS. The candidate shows typical LBV features in its spectra: broad and strong hydrogen lines and the He i lines with P Cygni profiles. Its remarkable spectral resemblance to the well known LBV P Cygni suggests a common nature of the objects and supports LBV classification of J004341.84+411112.0. We estimate the temperature, reddening, radius and luminosity of the star using its spectral energy distribution. Obtained bolometric luminosity of the candidate (M bol = −10.41 ± 0.12 mag) is quite similar to those of known LBV stars in the Andromeda galaxy. We analyzed a ten year light curve of the object in R filter. The candidate demonstrates photometric variations of the order of 0.4 mag, with an overall brightness increasing trend ΔR > 0.1 mag. Therewith, the corresponding color variations of the object are ...

Abstract. Based on 2MASS J and Ks photometry for the open star clusters NGC 2383, NGC 2384, Pismi... more Abstract. Based on 2MASS J and Ks photometry for the open star clusters NGC 2383, NGC 2384, Pismis 6, Pismis 8 and using color magnitude diagrams with isochrones fit, we found an age of log(age) = 8.3 (200±6Myr) for NGC 2383 and log(age) = 6.9 (8±6Myr) for NGC 2384. For Pismis 6 and Pismis 8 we adopted a range of log(age) = 6- 7 (1- 10 Myr). Because they similar ages, Pismis 6 and Pismis 8 may have been formed in the same Giant Molecular Cloud, and we concluded they are a good candidate for a binary system. In the case of NGC 2383 and NGC 2384, because the big age difference found we conclude that most probably they are born in different environments and as well are not physically connected. PACS number: 98.20.Di 1

GROUND-BASED ASTRONOMY IN RUSSIA. 21ST CENTURY. Proceedings of the All-Russian Conference, 2020

Calibrated Tully-Fisher relations ''absolute magnitude - HI line width'' for spir... more Calibrated Tully-Fisher relations ''absolute magnitude - HI line width'' for spiral galaxies oriented edge-on are derived from published data. The slope of the linear fits, especially in R- and I- bands, is close to the value of -10, as predicted by the theory. The standard errors of the relations in B, V, R and I bands are 0.40, 0.36, 0.31 and 0.34 mag, corresponding to relative distance errors of 18%, 17%, 14% and 16%, respectively. The magnitude of an edge-on galaxy appears to be a good distance indicator.

Monthly Notices of the Royal Astronomical Society, 2021

We report here the first results from a 15-yr long variability monitoring of the z = 2.0 quasar Q... more We report here the first results from a 15-yr long variability monitoring of the z = 2.0 quasar QSO B1312+7837. It shows luminosity changes with a period P ∼ 6.13 yr (P ∼ 2.04 yr at rest frame) and amplitude of ∼0.2 mag, superimposed on a gradual dimming at a rate of ∼0.55 mag per 100 yr. Two false periods associated with power peaks in the data windowing function were discarded. The measured period is confirmed with a bootstrapping Monte Carlo simulation. A damped random walk model yields a better fit to the data than a sine-function model, but at the cost of employing some high-frequency variations which are typically not seen in quasars. We consider the possible mechanisms driving this variability, and conclude that orbital motion of two supermassive black holes – result from a recent galaxy merger – is a possible explanation.

Astronomy & Astrophysics, 2015

Context. M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015-the first even... more Context. M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015-the first event of this kind observed in M 31 since 1988. Very few similar events have been confirmed as of 2015. Most of them are considered to be products of stellar mergers. Aims. Results of an extensive optical monitoring of the transient in the period January-March 2015 are presented. Methods. Eight optical telescopes were used for imaging. Spectra were obtained on the Large Altazimuth Telescope (BTA), the Gran Telecsopio Canarias (GTC) and the Rozhen 2 m telescope. Results. We present a highly accurate 70 d light curve and astrometry with a 0.05 uncertainty. The colour indices reached a minimum of 2−3 d before peak brightness and rapidly increased afterwards. The spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the next 30 d. The luminosity of the transient was estimated to be 8.7 +3.3 −2.2 × 10 5 L during the optical maximum. Conclusions. Both the photometric and the spectroscopic results confirm that the object is a red nova, similar to V838 Monocerotis.

We report the discovery of a probable nova located at RA(2000) = 00h43m52.94s, Dec(2000) = +41d03... more We report the discovery of a probable nova located at RA(2000) = 00h43m52.94s, Dec(2000) = +41d03'36", which is 12' 54" east and 12' 33" south of the center of M31. Available R-band magnitudes from 5x300sec and 3x300sec co-added CCD images taken with the 50/70cm Schmidt telescope (+ SBIG STL-11000M CCD camera) at NAO Rozhen, Bulgaria: November 6.809 UT, 19.7+/-0.2; 8.853

Several properties of the M31 disk, namely: opacity, extinction law and gas-to-dust ratio are stu... more Several properties of the M31 disk, namely: opacity, extinction law and gas-to-dust ratio are studied by means of optical and near-infrared photometry of ten globular clusters and galaxies seen through the disk. The individual extinctions of these objects were estimated with respect to several sets of theoretical spectral energy distributions for globulars and galaxies. Seven targets are consistent with reddened globulars, two - with starburst galaxies and one - with an elliptical. The extinction estimates agree with semi-transparent disk (Av ~ 1) in the inter-arm regions. The total-to-selective extinction ratio in those regions 2.75 is lower on average than the typical Galactic value of Rv=3.1. We also obtained a gas-to-dust ratio, similar to that in the the Milky way. It shows no correlation with the distance from the M31 center.

In this paper we examine the gas-to-dust ratio in M31 galaxy combining radio emission data on neu... more In this paper we examine the gas-to-dust ratio in M31 galaxy combining radio emission data on neutral hydrogen (HI) and carbon oxide (CO) with the recent extinction estimates of young stellar groups (OB associations and open clusters). The fraction of sightlines where the observed N(HI)/2EB-V ratio is outside the typical Galactic range is only 1/10; however, the correlation between these two quantities remains weak. A radial dependence of this ratio, twice flatter than previous estimates, indicates absence of a strong metallicity gradient. Detection of objects behind the disk of M31 allow pencil beams estimates of the total extinction. The results confirm the plausibility of a disk model with exponentially decreasing optical thickness in radial direction. Nearly constant gas−to−dust ratio in the Galactic interstellar medium in the few kpc vicinity of the Sun [1] shows that dust and gas are well mixed and the grain properties do not vary essentially. One comes to this conclusion taki...

Monthly Notices of the Royal Astronomical Society

We study five luminous blue variable (LBV) candidates in the Andromeda galaxy and one more (MN112... more We study five luminous blue variable (LBV) candidates in the Andromeda galaxy and one more (MN112) in the Milky Way. We obtain the same-epoch near-infrared (NIR) and optical spectra on the 3.5-m telescope at the Apache Point Observatory and on the 6-m telescope of the SAO RAS. The candidates show typical LBV features in their spectra: broad and strong hydrogen lines, He i, Fe ii, and [Fe ii] lines. We estimate the temperatures, reddening, radii and luminosities of the stars using their spectral energy distributions. Bolometric luminosities of the candidates are similar to those of known LBV stars in the Andromeda galaxy. One candidate, J004341.84+411112.0, demonstrates photometric variability (about 0.27 mag in the V band), which allows us to classify it as an LBV. The star J004415.04+420156.2 shows characteristics typical of B[e] supergiants. The star J004411.36+413257.2 is classified as a Fe ii star. We confirm that the stars J004621.08+421308.2 and J004507.65+413740.8 are warm hy...

Proceedings of the International Astronomical Union

We have compiled historical observations, spanning ∼100 years, for a dozen of the best-studied LB... more We have compiled historical observations, spanning ∼100 years, for a dozen of the best-studied LBVs in the Local Group. We described how we prepared structure functions for their light-curves and calculated two parameters (the structure function slope and the characteristic time-scale) to describe the behaviour of the LBVs. The sensitivity of those parameters to the variability behaviour of the stars was tested with a number of photometric data sets. The slope of the structure function may anticorrelate with the time-scale. These types of variable stellar objects are crucial to studies of stellar variability and the final stages of stellar evolution.

We derived individual extinction values for selected red supergiant (RSGs) candidates in M31 with... more We derived individual extinction values for selected red supergiant (RSGs) candidates in M31 with broadband photometry. Taking into account their position on the colour-magnitude diagram and using a probability method, the metallicity of each star was estimated. In the range 2-15 kpc the ratio [O/H] is nearly constant. The derived individual extinctions and pencil beams values from three different gas maps (Westerbork HI, VLA HI and IRAM CO (1→0)-line survey) were used to obtain gas-to-dust ratio in M31. For the 9 most luminous stars both the ratios N(HI)/2 EB-V and N(HI+H2)/2EB-V are not very different from those in the Milky Way. Significant fractions outside the expected range of Galactic atomic gas-to-dust ratio are obtained for ~1/3 of the sample using Westerbork and for ~1/2 of the sample using VLA maps. The ratios are overestimated for objects located high above the midplane of M31 and underestimated - due to resolution effects, - for several RSGs coinciding with small HI clouds.

We present a long-term optical search for novae in M31 galaxy, based on observations taken at the... more We present a long-term optical search for novae in M31 galaxy, based on observations taken at the 2m RCC telescope and the 50/70cm Schmidt telescope at NAO Rozhen, Bulgaria. Our monitoring of the M31 central region yields ˜ 15 per cent of all newly discovered novae during the last 5 years. Here we report coordinates and R-band magnitudes (BV-band if available) for 14 newly discovered nova candidates along with a Mira variable, mistaken for nova. Finding charts and light curves are presented. Times of decline (t_{2}) are estimated and Maximum Magnitude - Rate of Decline relationship (MMRD) is constructed.

An average value of the total-to-selective-extinction ratio R_V=3.8 +/- 0.4 in M31 is obtained by... more An average value of the total-to-selective-extinction ratio R_V=3.8 +/- 0.4 in M31 is obtained by means of two independent methods and by use of the analytical formula of Cardelli, Clayton & Mathis (1989). This result differs from previous determinations as well from the `standard' value 3.1 for the Milky Way. The derived individual extinctions for blue and red luminous stars from the catalogue of Magnier et al. (1992) are in good agreement with recent estimates for several OB associations in M31 and thus the issue about the assumed optical opacity of the spiral disk still remains open. The presented list of 113 red supergiant candidates in M31 with their extinctions and luminosities contains 60 new objects of this type which are not identified in other publications. It is supplemented with further 290 stars dereddened on the base of results for their closest neighbors. The luminosity function of all red supergiant candidates and the percentage of those with progenitors over 20 ...

Compact HII regions surrounding luminous early-type stars could be recognized by their contributi... more Compact HII regions surrounding luminous early-type stars could be recognized by their contribution to the measured magnitude Rc. Applying a special dereddening procedure to a sample with BVRcIc photometry from the catalog of Magnier et al. (1992), we selected starlike objects in M31 that are excessively bright in Rc due to Hα+[S II] emission. Our results are compared with the well calibrated flux densities of compact HII regions (Walterbos & Braun 1992). The typical luminosities (~ 1037 erg/s) and the estimated absolute average magnitudes of the embedded ionizing source (MV = -7.5m) point to a very massive single star or to a small cluster of several massive stars. We also derived the nominal internal extinction of 15 blue objects with reliable individual values of RV. The intrinsic extinction within HII regions is, typically, small indicating that the dust has been swept away by stellar winds.