Toward understanding the B[e] phenomenon: the curious case of IRAS 17449+2320

The existence of classical Be stars, stars that show emission lines due to a disk-like structure around the central object, has been recognize for more than one century. In contrast the history of the B[e] stars is much shorter. Conti suggested calling B-type stars that show low ionization forbidden emission lines as B[e]. Nowadays, we know that B[e] stars are an inhomogeneous group of objects so it is uncorrect to speak about a class of stars but it is better to talk about the B[e] phenomenon. The B[e] phenomenon appears in early-type emission lines stars with low-excitation lines, forbidden lines, and hot dust detected by an IR excess. The phenomenon is important because appears in very different evolutionary stages of stars (in the intermediate mass pre-main sequence stars HAeB[e], in the main sequence stars MSB[e], in the evolved high-mass stars sgB[e], evolved low-mass stars cPNB[e], symbiotic stars symB[e] etc.). Studying this phenomenon we can better understand the physical mechanisms that appear during the life of a star and correlations between dynamical mechanisms (wind, rotation, eruptions, non radial pulsations, binarity etc.) and evolution. It is important that B[e] objects harbor dust in a circumstellar disk and may be responsible for at least few percent of the Galactic circumstellar dust. Dust forming B[e] stars exist in the lower metallicity environments of LMC and SMC so it is reasonable to suggest that they might have been important dust producers in the earlier Universe. It is difficult to study these stars beacause they are embedded in a geometrically and optically thick circumstellar envelope, but, we can try to infer the physical properties by studying the interaction between the star and its environment. The object of my analysis is IRAS 17449+2320, it is a 10 mag B[e] object situated in the Hercules constellation at a distance of 700 pc from the Sun. IRAS 17449+2320 is catalogued as a A0V star and a possible FS CMa object, a subclass of the B[e] phenomenon. There are observational and physical criteria for the identification of a FS CMa star. Observational criteria: - Emission line spectra contain H lines stronger than in Be, HAeBe, and normal supergiants; Fe II and [O I]; sometimes [Fe II] and weak [O III] lines. -Large IR excess that peaks at 10-30 micrometer and sharply decreases longward. -Location outside of star forming regions. -When presents, a secondary companion (i) is typically fainter and cooler (if it is a normal star) than the primary or (ii) is degenerate. Physical criteria: -T_eff of the hot star/companion of about 9000- 30000 K (O9 - A2 type stars). -Luminosity of the hot star/companion log (L/L_sun) approx 2.5 - 4.5 V_pri -V_sec > 2 mag This is an observational thesis, so a special focus is on the data analysis. I will explain how to reduce spectral data, both slit and échelle, with IRAF software, then I will analyze the spectra taken from the Ondrejov Observatory, the Observatorio Astronomico Nacional San Pedro Martir , McDonald Observatory and the Canada France Hawaii Telescope. In particular, I will analyze line profile variations, expecially for the Balmer lines. Then, theoretical chapters follow, trying to describe the phenomenology behind the physical mechanisms inferred by the spectroscopic studies. The Balmer lines have broad wings in absorption wide 3600 km/s (FWZI) but the core is composite by emission and absorption features. The variation of Balmer lines is only in the core and in the range of velocity of [-300,300] km/s, this means that the dynamical phenomena that are affecting these lines are playing at a distance lesser than an AU. The wings of Fe II, Na I D, O I and Ca II are broad as the range of velocity where the dynamical phenomena are affecting the Balmer lines. This can suggest that these lines are forming in the same region of the perturbations. Instead the width of [O I] emission lines suggest that [O I] are forming in a wider low density region and it is possible to use the doppler shift of these lines as an indicator of the radial velocity of the system that is RV=(-16\pm 2) km/s. I found in the Balmer lines, the presence of composite absorption features moving from the blue to the red side with a time scale of few days, changing their velocity of 200 km/s. This behaviour be explained by the presence of coherent but temporary structures that are orbiting around the star at about 0.2 AU where the radiation has a temperature of about approx 2900 K. The origin of this structure is not known, but some scenarios are the presence of global one-armed (i.e. azimuthal wave number m = 1) oscillations in the equatorial discs and the evaporation of a planet forming a multi-cometary tail. Line variations are found on a range of timescales: few days for Balmer lines, and month or years for the other lines. The presence of He I lines suggests that the star is earlier than thought by Miroshnichenko but not earlier than a B8V. The wings of Balmer lines compared with the ones obtained rotating the spectrum of Vega gives an estimation of the projected rotational velocity. The best match is assuming v_{eq} sin(theta) approx 200 km/s, hence the star is a fast rotator. FS CMa objects sometimes show binarity, in this case there is no clear evidence of a companion but binarity cannot be ruled out since a brown dwarf or a white dwarf do not produce detectable spectra

The Pristine Dwarf-Galaxy survey -- V. The edges of the dwarf galaxy Hercules

We present a new spectroscopic study of the dwarf galaxy Hercules (d ~ 132 kpc) with data from the Anglo-Australian Telescope and its AAOmega spectrograph together with the Two Degree Field multi-object system to solve the conundrum that whether Hercules is tidally disrupting. We combine broadband photometry, proper motions from Gaia, and our Pristine narrow-band and metallicity-sensitive photometry to efficiently weed out the Milky Way contamination. Such cleaning is particularly critical in this kinematic regime, as both the transverse and heliocentric velocities of Milky Way populations overlap with Hercules. Thanks to this method, three new member stars are identified, including one at almost 10rh of the satellite. All three have velocities and metallicities consistent with that of the main body. Combining this new dataset with the entire literature cleaned out from contamination shows that Hercules does not exhibit a velocity gradient (d/dX = 0.1+0.4/-0.2 km s-1 arcmin-1) and, as such, does not show evidence to undergo tidal disruption

Small-scale stellar haloes: detecting low surface brightness features in the outskirts of Milky Way dwarf satellites

Dwarf galaxies are valuable laboratories for dynamical studies related to dark matter and galaxy evolution, yet it is currently unknown just how physically extended their stellar components are. Satellites orbiting the Galaxy's potential may undergo tidal stripping by the host, or alternatively, may themselves have accreted smaller systems whose debris populates the dwarf's own stellar halo. Evidence of these past interactions, if present, is best searched for in the outskirts of the satellite. However, foreground contamination dominates the signal at these large radial distances, making observation of stars in these regions difficult. In this work, we introduce an updated algorithm for application to Gaia data that identifies candidate member stars of dwarf galaxies, based on spatial, color-magnitude and proper motion information, and which allows for an outer component to the stellar distribution. Our method shows excellent consistency with spectroscopically confirmed members from the literature despite having no requirement for radial velocity information. We apply the algorithm to all $\sim$60 Milky Way dwarf galaxy satellites, and we find 9 dwarfs (Bo\"otes 1, Bo\"otes 3, Draco 2, Grus 2, Segue 1, Sculptor, Tucana 2, Tucana 3, and Ursa Minor) that exhibit evidence for a secondary, low-density outer profile. We identify many member stars which are located beyond 5 half-light radii (and in some cases, beyond 10). We argue these distant stars are likely tracers of dwarf stellar haloes or tidal streams, though ongoing spectroscopic follow-up will be required to determine the origin of these extended stellar populations.Comment: 24 pages, 13 figures, 4 tables, submitted to MNRA

Stars on the edge: Galactic tides and the outskirts of the Sculptor dwarf spheroidal

Stars far beyond the half-light radius of a galaxy suggest the existence of a mechanism able to move stars out of the region where most star formation has taken place. The formation of these "stellar halos" are usually ascribed to the effects of early mergers or Galactic tides, although fluctuations in the gravitational potential due to stellar feedback is also a possible candidate mechanism. A Bayesian algorithm is used to find new candidate members in the extreme outskirts of the Sculptor dwarf galaxy. Precise metallicities and radial velocities for two distant stars are measured from their spectra taken with the Gemini South GMOS spectrograph. The radial velocity, proper motion and metallicity of these targets are consistent with Sculptor membership. As a result, the known boundary of the Sculptor dwarf extends now out to an elliptical distance of $\sim10$ half-light radii, which corresponds to a projected physical distance of $\sim3$ kpc. As reported in earlier work, the overall distribution of radial velocities and metallicities indicate the presence of a more spatially and kinematically dispersed metal-poor population that surrounds the more concentrated and colder metal-rich stars. Sculptor's density profile shows a "kink" in its logarithmic slope at a projected distance of $\sim25$ arcmin (620 pc), which we interpret as evidence that Galactic tides have helped to populate the distant outskirts of the dwarf. We discuss further ways to test and validate this tidal interpretation for the origin of these distant stars.Comment: 10 pages, 4 figures, submitted to MNRA

Could very low-metallicity stars with rotation-dominated orbits have been shepherded by the bar?

The most metal-poor stars (e.g. [Fe/H] $\leq-2.5$) are the ancient fossils from the early assembly epoch of our Galaxy, very likely before the formation of the thick disc. Recent studies have shown that a non-negligible fraction of them have prograde planar orbits, which makes their origin a puzzle. It has been suggested that a later-formed rotating bar could have driven these old stars from the inner Galaxy outward, and transformed their orbits to be more rotation-dominated. However, it is not clear if this mechanism can explain these stars as observed in the solar neighborhood. In this paper, we explore the possibility of this scenario by tracing these stars backwards in an axisymmetric Milky Way potential with a bar perturber. We integrate their orbits backward for 6 Gyr under two bar models: one with a constant pattern speed and another one with a decelerating speed. Our experiments show that, under the constantly-rotating bar model, the stars of interest are little affected by the bar and cannot have been shepherded from a spheroidal inner Milky Way to their current orbits. In the extreme case of a rapidly decelerating bar, some of the very metal-poor stars on planar and prograde orbits can be brought from the inner Milky Way, but $\sim90\%$ of them were nevertheless already rotation-dominated ($J_{\phi}$ $\geq$ 1000 km s$^{-1}$ kpc) 6 Gyr ago. The chance of these stars having started with spheroid-like orbits with small rotation ($J_{\phi}$ $\lesssim$ 600 km s$^{-1}$ kpc) is very low ($<$ 3$\%$). We therefore conclude that, within the solar neighborhood, the bar is unlikely to have shepherded a significant fraction of inner Galaxy spheroid stars to produce the overdensity of stars on prograde, planar orbits that is observed today.Comment: submitted to A&A, comments are welcom

Discovery of a new Local Group Dwarf Galaxy Candidate in UNIONS: Bo\"otes V

We present the discovery of Bo\"otes V, a new ultra-faint dwarf galaxy candidate. This satellite is detected as a resolved overdensity of stars during an ongoing search for new Local Group dwarf galaxy candidates in the UNIONS photometric dataset. It has a physical half-light radius of 26.9$^{+7.5}_{-5.4}$ pc, a $V$-band magnitude of $-$4.5 $\pm$ 0.4 mag, and resides at a heliocentric distance of approximately 100 kpc. We use Gaia DR3 astrometry to identify member stars, characterize the systemic proper motion, and confirm the reality of this faint stellar system. The brightest star in this system was followed up using Gemini GMOS-N long-slit spectroscopy and is measured to have a metallicity of [Fe/H] $=$ $-$2.85 $\pm$ 0.10 dex and a heliocentric radial velocity of $v_r$ = 5.1 $\pm$ 13.4 km s$^{-1}$. Bo\"otes V is larger (in terms of scale radius), more distant, and more metal-poor than the vast majority of globular clusters. It is likely that Bo\"otes V is an ultra-faint dwarf galaxy, though future spectroscopic studies will be necessary to definitively classify this object.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in the AAS Journals. Please note that this paper was submitted in coordination with the work of William Cerny et al. 2022. These authors independently discovered this same satellite so our two research groups have coordinated the submission of these discovery paper

The discovery of the faintest known Milky Way satellite using UNIONS

We present the discovery of Ursa Major III/UNIONS 1, the least luminous known satellite of the Milky Way, which is estimated to have an absolute V-band magnitude of $+2.2^{+0.4}_{-0.3}$ mag, equivalent to a total stellar mass of 16$^{+6}_{-5}$ M$_{\odot}$. Ursa Major III/UNIONS 1 was uncovered in the deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is consistent with an old ($\tau > 11$ Gyr), metal-poor ([Fe/H] $\sim -2.2$) stellar population at a heliocentric distance of $\sim$ 10 kpc. Despite being compact ($r_{\text{h}} = 3\pm1$ pc) and composed of so few stars, we confirm the reality of Ursa Major III/UNIONS 1 with Keck II/DEIMOS follow-up spectroscopy and identify 11 radial velocity members, 8 of which have full astrometric data from $Gaia$ and are co-moving based on their proper motions. Based on these 11 radial velocity members, we derive an intrinsic velocity dispersion of $3.7^{+1.4}_{-1.0}$ km s$^{-1}$ but some caveats preclude this value from being interpreted as a direct indicator of the underlying gravitational potential at this time. Primarily, the exclusion of the largest velocity outlier from the member list drops the velocity dispersion to $1.9^{+1.4}_{-1.1}$ km s$^{-1}$, and the subsequent removal of an additional outlier star produces an unresolved velocity dispersion. While the presence of binary stars may be inflating the measurement, the possibility of a significant velocity dispersion makes Ursa Major III/UNIONS 1 a high priority candidate for multi-epoch spectroscopic follow-ups to deduce to true nature of this incredibly faint satellite.Comment: 21 pages, 9 figures, 3 tables; Accepted for publication in Ap