A. Tornambe

The Astronomical Observatory of Brera, Merate, Lombardy, Italy

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Publications (75)195.08 Total impact

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    A. Tornambé, L. Piersanti
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    ABSTRACT: The evolutionary path of rotating CO WDs directly accreting CO-rich matter is followed up to few seconds before the explosive breakout in the framework of the Double Degenerate rotationally-driven accretion scenario. We find that the evolutionary properties depend only on the actual mass of the accreting WD and not on the previous history. We determine the expected frequency and amplitude of the gravitational wave emission, which occurs during the mass transfer process and acts as a self-tuning mechanism of the accretion process itself. The gravitational signal related to Galactic sources can be easily detected with the next generation of space-born interferometers and can provide notable constraints to the progenitor model. The expected statistical distribution of pre-explosive objects in the Galaxy is provided also in the effective temperature-apparent bolometric magnitude diagrams which can be used to identify merged DD systems via UV surveys. We emphasize that the thermonuclear explosion occurs owing to the decay of physical conditions keeping over-stable the structure above the classical Chandrasekhar limit and not by a steady increase of the WD mass up to this limit. This conclusion is independent of the evolutionary scenario for the progenitors, but it is a direct consequence of the stabilizing effect of rotation. Such an occurrence is epistemological change of the perspective in defining the ignition process in accreting WDs. Moreover, this requires a long evolutionary period (several million years) to attain the explosion after the above mentioned conditions cease to keep stable the WD. Therefore it is practically impossible to detect the trace of the exploding WD companion in recent pre-explosion frames of even very near SN Ia.
    Monthly Notices of the Royal Astronomical Society 04/2013; 431(2). · 5.52 Impact Factor
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    ABSTRACT: We calculate the dust formed around AGB and SAGB stars of metallicity Z=0.008 by following the evolution of models with masses in the range 1M<M<8M throughthe thermal pulses phase, and assuming that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface. We find that, because of the strong Hot Bottom Burning (HBB) experienced, high mass models produce silicates, whereas lower mass objects are predicted to be surrounded by carbonaceous grains; the transition between the two regimes occurs at a threshold mass of 3.5M. These fndings are consistent with the results presented in a previous investigation, for Z=0.001. However, in the present higher metallicity case, the production of silicates in the more massive stars continues for the whole AGB phase, because the HBB experienced is softer at Z=0.008 than at Z=0.001, thus the oxygen in the envelope, essential for the formation of water molecules, is never consumed completely. The total amount of dust formed for a given mass experiencing HBB increases with metallicity, because of the higher abundance of silicon, and the softer HBB, both factors favouring a higher rate of silicates production. This behaviour is not found in low mass stars,because the carbon enrichment of the stellar surface layers, due to repeated Third Drege Up episodes, is almost independent of the metallicity. Regarding cosmic dust enrichment by intermediate mass stars, we ?nd that the cosmic yield at Z=0.008 is a factor 5 larger than at Z=0.001. In the lower metallicity case carbon dust dominates after about 300 Myr, but at Z=0.008 the dust mass is dominated by silicates at all times,with a prompt enrichment occurring after about 40 Myr, associated with the evolution of stars with masses M =7.5 -8M.
    Monthly Notices of the Royal Astronomical Society 05/2012; 424(3). · 5.52 Impact Factor
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    ABSTRACT: We compute the mass and composition of dust produced by stars with masses in the range ? and with a metallicity of Z= 0.001 during their asymptotic giant branch (AGB) and super-AGB phases. Stellar evolution is followed from the pre-main-sequence phase using the code ATON which provides, at each time-step, the thermodynamics and the chemical structure of the wind. We use a simple model to describe the growth of the dust grains under the hypothesis of a time-independent, spherically symmetric stellar wind. Although part of the modelling which describes the stellar outflow is not completely realistic, this approach allows a straight comparison with results based on similar assumptions present in the literature, and thus can be used as an indication of the uncertainties affecting the theoretical investigations focused on the dust formation process in the surroundings of AGB stars. We find that the total mass of dust injected by AGB stars in the interstellar medium does not increase monotonically with stellar mass and ranges between a minimum of ? for the 1.5-? stellar model up to ?, for the 6-? case. Dust composition depends on the stellar mass: low-mass stars (?) produce carbon-rich dust, whereas more massive stars, experiencing Hot Bottom Burning, never reach the C-star stage, and produce silicates and iron. This is in partial disagreement with previous investigations in the literature, which are based on synthetic AGB models and predict that, when the initial metallicity is Z= 0.001, carbon-rich dust is formed at all stellar masses. The differences are due to the different modelling of turbulent convection in the super-adiabaticity regime. Also in this case, like for other physical features of the AGB, the treatment of super-adiabatic convection shows up as the most relevant issue affecting the dust formation process. We also investigate super-AGB stars with masses in the range ? that evolve over an ONe core. Due to a favourable combination of mass-loss and Hot Bottom Burning, these stars are predicted to be the most efficient silicate-dust producers, releasing ? masses of dust. We discuss the robustness of these predictions and their relevance for the nature and evolution of dust at early cosmic times.
    Monthly Notices of the Royal Astronomical Society 02/2012; 420(2):1442-1456. · 5.52 Impact Factor
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    ABSTRACT: We compute the mass and composition of dust produced by stars with masses in the range 1Msun<M<8 Msun and with a metallicity of Z=0.001 during their AGB and Super AGB phases. Stellar evolution is followed from the pre-main sequence phase using the code ATON which provides, at each timestep, the thermodynamics and the chemical stucture of the wind. We use a simple model to describe the growth of the dust grains under the hypothesis of a time-independent, spherically symmetric stellar wind. We find that the total mass of dust injected by AGB stars in the interstellar medium does not increase monotonically with stellar mass and ranges between a minimum of 10^{-6}Msun for the 1.5Msun stellar model, up to 2x10^{-4} Msun, for the 6Msun case. Dust composition depends on the stellar mass: low-mass stars (M < 3Msun) produce carbon-rich dust, whereas more massive stars, experiencing Hot Bottom Burning, never reach the carbon-star stage, and produce silicates and iron. This is in partial disagreement with previous investigations in the literature, which are based on synthetic AGB models and predict that, when the initial metallicity is Z=0.001, C-rich dust is formed at all stellar masses. The differences are due to the different modelling of turbulent convection in the super-adiabaticity regime. Also in this case the treatment of super-adiabatic convection shows up as the most relevant issue affecting the dust-formation process. We also investigate Super AGB stars with masses 6.5Msun<M<8 Msun that evolve over a ONe core.Due to a favourable combination of mass loss and Hot Bottom Burning, these stars are predicted to be the most efficient silicate-dust producers, releasing [2 - 7]x 10^{-4} Msun masses of dust. We discuss the robustness of these predictions and their relevance for the nature and evolution of dust at early cosmic times.
    11/2011;
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    ABSTRACT: A decade ago the observations of thermonuclear supernovae at high-redhifts showed that the expansion rate of the Universe is accelerating and since then, the evidence for cosmic acceleration has gotten stronger. This acceleration requires that the Universe is dominated by dark energy, an exotic component characterized by its negative pressure. Nowadays all the available astronomical data (i.e. thermonuclear supernovae, cosmic microwave background, barionic acoustic oscillations, large scale structure, etc.) agree that our Universe is made of about 70% of dark energy, 25% of cold dark matter and only 5% of known, familiar matter. This Universe is geometrically flat, older than previously thought, its destiny is no longer linked to its geometry but to dark energy, and we ignore about 95% of its components. To understand the nature of dark energy is probably the most fundamental problem in physics today. Current astronomical observations are compatible with dark energy being the vacuum energy. Supernovae have played a fundamental role in modern Cosmology and it is expected that they will contribute to unveil the dark energy. In order to do that it is mandatory to understand the limits of supernovae as cosmological distance indicators, improving their precision by a factor 10.
    08/2009;
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    ABSTRACT: In the framework of the rotating Double Degenerate Scenario for type Ia Supernovae progenitors, we show that the dichotomy between explosive events in early and late type galaxies can be easily explained. Assuming that more massive progenitors produce slow-decline (high-luminosity) light curve, it comes out that, at the current age of the Universe, in late type galaxies the continuous star formation provides very massive exploding objects (prompt component) corresponding to slow-decline (bright) SNe; on the other hand, in early type galaxies, where star formation ended many billions years ago, only low mass ``normal luminosity'' objects (delayed component) are present.
    AIP Conference Proceedings. 05/2009; 1111(1).
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    ABSTRACT: We show that, by assuming the rotating Double Degenerate scenario for the SNe Ia progenitors, the total mass of the progenitor systems is the leading parameter determining the observational properties of these events. Moreover, basing on population synthesis models, we show that the distribution of DD systems good candidate as SNe Ia progenitors in late and early type galaxies well explains the evidence that high luminosity events occurs only in star forming galaxies.
    Memorie della Societa Astronomica Italiana. 01/2009;
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    ABSTRACT: Nowadays it is widely accepted that the current Universe is dominated by dark energy and exotic matter, the so called StandardModel of Cosmoloy or CDM model. All the available data (Thermonuclear Supernovae, Cosmic Microwave Background, Baryon Acoustic Oscillations, Large Scale Structure, etc.) are compatible with a flat Universe made by ~70% of dark energy. Up to now observations agree that dark energy may be the vacuum energy (or cosmological constant) although improvements are needed to constrain further its equation of state. In this context, the cosmic destiny of the Universe is no longer linked to its geometry but to the nature of dark energy; it may be flat and expand forever or collapse. To understand the nature of dark energy is probably the most fundamental problem in physics today; it may open new roads of knowledge and led to unify gravity with the other fundamental interactions in nature. It is expected that astronomical data will continue to provide directions to theorists and experimental physicists. Type Ia supernovae (SNe Ia) have played a fundamental role, showing the acceleration of the expansion rate of the Universe a decade ago, and up to now they are the only astronomical observations that provide a direct evidence of the acceleration. However, in order to determine the source of the dark energy term it is mandatory to improve the precision of supernovae as distance indicators on cosmological scale.
    Lecture Notes and Essays in Astrophysics. 11/2008; 3:45-62.
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    ABSTRACT: The accuracy needed to identify the nature of dark energy in the Universe is one order of magnitude smaller than the present scatter in the observed local relation used to calibrate Type Ia SNe as standard candles. In order to reduce this scatter and to identify potential evolutionary effects, we have studied the influence of the physical and chemical properties of progenitors on SNe Ia light curves. First, we considered the mass and composition of WD progenitors and then we took into account rotation during the WD accretion phase, obtaining super‐Chandrasekhar mass WDs. Differences in the maximum magnitude are of the same order as the present scatter in the calibration relation. In the context of the one dimensional delayed detonation model we assume the same transition density in all our models, implying that similar amounts of radioactive nickel are synthesized and, thus, similar magnitudes at the maximum epoch of the light curve are obtained. To establish, from a theoretical point of view, the required links between progenitors and outcomes, we have to fully understand the physics of Type Ia SNe, otherwise we may have already reached their limit as a probe of the structure of the Universe. © 2007 American Institute of Physics
    AIP Conference Proceedings. 08/2007; 924(1):218-225.
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    01/2007;
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    A. Tornambé, L. Piersanti
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    ABSTRACT: We show that in the framework of canonical stellar evolution it is hard, if not impossible, to determine the growth in mass of a CO White Dwarf, member of a binary system, up to the Chandrasekhar limit. This is the case either if matter is accreted from an H-rich companion or if direct CO accretion occurs from a CO-WD companion. At contrast, we show that by including the effects of rotation in modeling the accretion process, a CO-WD can increase its mass at the expenses of the degenerate CO companion up and beyond M_{Ch}, and that an explosive event of the type Ia class is naturally produced. This theoretical finding revives the Double Degenerate scenario for type Ia Supernova progenitors. In such a case the internal spread in the observational properties of type Ia Supernovae may be interpreted as a consequence of different total masses; hence differences between SNe Ia in nearby elliptical galaxies and the majority of those in spirals should be expected and the current use of type Ia SNe as cosmological distance indicators should be justified.
    11/2005; 342:169.
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    ABSTRACT: High redshift SNe Ia have been recently used to calibrate the cosmological distance scale and to infer the existence of the dark energy. The reliability of such a method depends on the effective knowledge of the absolute brightness of this class of supernovae. This would require a complete understanding of the physics of SNeIa.
    AIP Conference Proceedings. 10/2005; 797(1):497-503.
  • L. Piersanti, A. Tornambé, V. Castellani
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    ABSTRACT: This paper addresses the problem of pseudo-evolutionary zero-age horizontal branch (ZAHB) models, as produced in the current literature, starting from suitable assumptions about the post-He flash structures. To test the accuracy of this procedure, we present evolutionary computations of 0.9-M⊙ stellar models from their pre-main sequence through H burning and the He flash phases until the end of quiescent central He burning, for selected assumptions about the star metallicity and the amount of mass lost before the onset of He ignition. The effects of mass loss on the evolution of low-mass red giants are shortly revisited, discussing in some detail the evolution from the red giant tip to the onset of quiescent He burning as well as the chemical structure of the newborn HB stars. We find that the procedure adopted to produce self-consistent pseudo-evolutionary ZAHB models appears supported by detailed computations covering a large range of He flashing structures for different assumptions on both the rate of mass loss and the star metallicity. By comparing the evolution of both types of models through the whole phase of central He burning, we conclude that pseudo-evolutionary ZAHB models represent an excellent approximation of the actual behaviour of low-mass He-burning structures. As a relevant point, we find that no ‘blue nose’ is expected at the end of the pre-HB evolutionary path and, as a consequence, that pre-HB evolution does not play any role in the modal stability of RR Lyrae pulsators.
    Monthly Notices of the Royal Astronomical Society 08/2004; 353(1):243 - 248. · 5.52 Impact Factor
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    ABSTRACT: We present a promising scenario which removes several main concerns related to supernova-like outcomes in double degenerate binary systems which merge. Rotation, which occurs naturally in stellar components of these systems, has been taken into account during the entire evolutionary process (i.e., pre-merging evolution, accretion phase, and final establishment of the accreting structure as a dynamically explosive configuration at the rotational Chandrasekhar mass value). The physical properties of the system are now such that a self-regulating accretion process (which operates until the mass of the accreting white dwarf component reaches the rotational Chandrasekhar mass limit) becomes unavoidable, as does the occurrence of central carbon ignition and an explosive outcome of supernova Ia proportions. We are optimistic that further investigations of this scenario will lead naturally to the (so far missing) correct explanation of observational properties of SN Ia events.
    01/2004;
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    ABSTRACT: If the lifting effect of rotation is taken into account in modeling the evolution of CO WDs accreting CO-rich matter as a consequence of a merging process, it turns out that rotation triggers the accretion rate in such a way that the WD can increase in mass up and over the Chandrasekhar limit. In this case the braking due to the viscous friction between the accretion disk and the WD as well as to the possible GWR emission produces the contraction of the accreting structure so that the physical conditions suitable for explosive Carbon burning are attained at the center. These results are valid for WDs with total masses in the range 1.4/1.5 M_&sun;, independently on the braking efficiency.
    Memorie della Societa Astronomica Italiana. 01/2003;
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    ABSTRACT: IAUC 7901 available at Central Bureau for Astronomical Telegrams.
    International Astronomical Union Circular. 05/2002;
  • L. Piersanti, S. Gagliardi, A. Tornambé
    04/2002;
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    ABSTRACT: We present the preliminary results of near-infrared photometric observations of the type IIn SN1999EL covering the period between 5 to 80 days after discovery. These infrared data represeat the second time that such extensive time and wavelength coverage has been achieved for a type IIn SN event. We estimate the absolute magnitudes of the SN at the moment of brightness maximum in J, H and K bands. If the luminosity of the SN event in the K band is powered mostly by the deposition of redioactive decay energy, the mass of radioactive Ni is estimated as 0.2M⊙. The effect of blueing of the J, H, K light curves is discovered and explained by the interaction of the explosion shock and UV/optical flash with pre-existing dust in the circumstellar medium of the progenitor.
    Astronomical and Astrophysical Transactions 01/2001; 20(2):385-391.
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    ABSTRACT: During the preliminary phases of our project we have decided to systematically observe the type IIn supernova SN1999el discovered in the spiral galaxy NGC 6951 by Cao et al. (IAUC 7288, 1999). This supernova has been chosen because well sampled near-IR light curves of a type IIn are not available in literature so far. During this follow-up we have discovered, in the same galaxy, a type Ia new supernova, named SN2000E (Valentini el al. 2000, IAUC 7351). We present here the preliminary near-IR/optical light curves of both supernovae.
    Memorie della Societa Astronomica Italiana. 01/2001; 72:883-887.
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    L. Piersanti, S. Cassisi, A. Tornambe
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    ABSTRACT: The role played by NCO reaction in the onset of the He flash in the He core of a RGB star and in low mass CO WDs accreting H/He rich matter is investigated.
    06/2000;

Publication Stats

465 Citations
195.08 Total Impact Points

Institutions

  • 1989–2007
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 1984–1989
    • National Institute of Astrophysics
      Roma, Latium, Italy
    • Niigata University
      Niahi-niigata, Niigata, Japan
  • 1984–1986
    • European Southern Observatory
      Arching, Bavaria, Germany