arXiv subjects

Sergio A. Cellone

Publications and source records attributed to Sergio A. Cellone.

23 records · Page 2Linked to original sources

arXiv2006

Extremely violent optical microvariability in blazars: fact or fiction?

Variability amplitudes larger than 1 magnitude over time-scales of a few tens of minutes have recently been reported in the optical light-curves of several blazars. In order to independently verify the real occurrence of such extremely violent events, we undertook an observational study of a selected sample of three blazars: PKS 0048-097, PKS 0754+100, and PKS 1510-089. Possible systematic error sources during data acquisition and reduction were carefully evaluated. We indeed found flux variability at intra-night time-scales in all three sources, although no extremely violent behaviour, as reported by other authors, was detected. We show that an incorrect choice of the stars used for differential photometry will, under fairly normal conditions, lead to spurious variability with large amplitudes on short time-scales. Wrong results of this kind can be avoided with the use of simple error-control techniques.

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arXiv2006

The low-luminosity galaxy population in the NGC 5044 Group (Conference proceeding)

Detailed surface photometry for 79 (mostly dwarf) galaxies in the NGC 5044 Group area is analysed, revealing the existence of different morphologies among objects originally classified as early-type dwarfs. Particularly, a significant fraction of bright dwarf "ellipticals" show a distinct bulge+disc structure; we thus re-classify these objects as dwarf lenticulars (dS0). Our finding points at a possible scenario where these systems are the remnants of "harassed" disc galaxies. This is emphasized by the discovery of a few objects with hints for very low-surface brightness spiral-like structure. The colours, structure, and spatial distribution of the different galaxy types suggest that our classification may indeed be separating objects with different origins and/or evolutionary paths.

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arXiv2004

The low-luminosity galaxy population in the NGC 5044 Group

We present multicolour imaging for 33 dwarf and intermediate-luminosity galaxies in the field of the NGC 5044 Group, complemented with mid-resolution spectroscopy for a subsample of 13 objects. With these data, a revised membership and morphological classification is made for the galaxies in the sample. We were able to confirm all but one of the "definite members" included in the spectroscopic subsample, which were originally classified based on morphological criteria; however, an important fraction of background galaxies is probably present among "likely" and "possible" members. The presence of a nucleus could be detected in just five out of the nine galaxies originally classified as dE,N, thus confirming the intrisic difficulty of photographic-plate morphological classification for this kind of object. Our deep surface photometry provided clear evidences for disc structure in at least three galaxies previously catalogued as dE or dS0. Their transition-type properties are also evident from the colour-magnitude diagram, where they lie near the late-type galaxies locus, suggesting an evolutionary connection between a parent disc-galaxy population and at least part of present-day dEs. Six new dSph candidates were also found, most of them at small projected distances from NGC 5044, the central galaxy of the Group. The NGC 5044 Group appears clearly defined in redshift space, with a mean heliocentric radial velocity, =2461 +/- 84 km/s (z=0.0082), and a moderate dispersion, σ_{v_r}=431 km/s. Our data show no luminosity segregation for early-type galaxies: both dwarf and bright E/S0 systems show very similar velocity distributions (σ_{v_r} ~ 290 km/s), in contrast to late-type galaxies that seem to display a broader distribution (σ_{v_r} ~ 680 km/s).

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arXiv2002

A Polarimetric Survey for Dust in 47 Tucanae (NGC 104)

We present linear polarization in the V band for 77 stars in the field of the globular cluster 47 Tucanae (NGC 104), and for 14 bright-star-free regions, located along an elliptical isophotal contour of the cluster, as well as UBVRI measurements for the cluster nucleus. The observations show variable foreground polarization that, once removed, leaves marginally significant polarization residuals for the non-variable bright red giants. Although these residuals are small there is, however, a systematic trend in the sense that the larger ones are seen towards the south of the cluster (in a direction opposite to that of the cluster proper motion). In contrast, most of the variable stars do show significant intrinsic polarization. The behavior of the star-free regions is similar to that of the non-variable stars and sets an upper limit to the possible existence of a global pattern of scattered (and polarized) intra-cluster light in the V band. In turn, the multicolor observations of the cluster nucleus cannot be fitted with a Serkowski law and exhibit a polarization excess both in U and B. This polarization could be explained as a combination of the foreground interstellar component and another one arising in dust located in the nucleus and illuminated by a bright blue post-AGB star (at 48" from the cluster center). An inspection of a set of archive HST WFPC2 images reveals the presence of a number of dark patches in the innermost regions of the cluster. A prominent patch (some 5" by 3" in size) located at 12" from the cluster center and with a position angle (N to E) of 120 degrees, has a slightly different polarization, compared to that of the cluster nucleus, and appears as a good candidate to be identified as a dust globule within the cluster.

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arXiv2001

Cometary activity in 2060 Chiron at minimum brightness

We present two-colour CCD imaging of 2060 Chiron obtained between 1996 and 1998 with the 2.15 m telescope at CASLEO (San Juan, Argentina). These post-perihelion observations show that Chiron was then near its historical brightness minima, however a coma was clearly detected. The dynamical state of the coma is studied by means of azimuthally averaged surface brightness profiles, which show the signatures of radiation pressure on the dust grain distribution. Aperture photometry shows an achromatic dimming with an amplitude \~ 0.09 mag in approximately one hour. If due to rotation of the nucleus, this rather high amplitude is used to derive a new value for the nuclear magnitude, m_0 ~ 6.80 mag.

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