arXiv subjects

D. Minniti

Publications and source records attributed to D. Minniti.

At least 73 records · Page 4Linked to original sources

arXiv2021

Analysis of physical processes in eruptive YSOs with near infrared spectra and multi-wavelength light curves

The decade-long Vista Variables in the Via Lactea (VVV) survey has detected numerous highly variable young stellar objects (YSOs). We present a study of 61 highly variable VVV YSOs ($ΔK_s$ = 1-5 mag), combining near infrared spectra from Magellan and VLT with VVV and NEOWISE light curves to investigate physical mechanisms behind eruptive events. Most sources are spectroscopically confirmed as eruptive variables (typically Class I YSOs) but variable extinction is also seen. Among them, magnetically controlled accretion, identified by H{\sc i} recombination emission (usually accompanied by CO emission), is observed in 46 YSOs. Boundary layer accretion, associated with FU Ori-like outbursts identified by CO overtone and H$_2$O absorption, is observed only in longer duration events ($\ge$5 yr total duration). However, even in long duration events, the magnetically controlled accretion mode predominates, with amplitudes similar to the boundary layer mode. Shorter (100-700 days) eruptive events usually have lower amplitudes and these events are generally either periodic accretors or multiple timescale events, wherein large photometric changes occur on timescales of weeks and years. We find that the ratio of amplitudes in $K_s$ and $W2$ can distinguish between variable accretion and variable extinction. Several YSOs are periodic or quasi-periodic variables. We identify examples of periodic accretors and extinction-driven periodicity among them (with periods up to 5 yr) though more data are needed to classify some cases. The data suggest that dynamic interactions with a companion may control the accretion rate in a substantial proportion of eruptive systems, although star-disc interactions should also be considered.

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arXiv2021

APOGEE view of the globular cluster NGC 6544

The second phase of the APOGEE survey is providing near-infrared, high-resolution, high signal-to-noise spectra of stars in the halo, disk, bar and bulge of the Milky Way. The near-infrared spectral window is especially important in the study of the Galactic bulge, where stars are obscured by the dust and gas of the disk in its line-of-sight. We present a chemical characterisation of the globular cluster NGC 6544 with high-resolution spectroscopy. The characterisation of the cluster chemical fingerprint, given its status of "interloper" towards the Galactic bulge and clear signatures of tidal disruption in its core is crucial for future chemical tagging efforts. Cluster members were selected from the DR16 of the APOGEE survey, using chemo-dynamical criteria of individual stars. A sample of 23 members of the cluster was selected. An analysis considering the intra-cluster abundance variations, known anticorrelations is given. According to the RGB content of the cluster, the iron content and $α$-enhancement are [Fe/H] $= -1.44 \pm 0.04$ dex and [$α$/Fe] $= 0.20 \pm 0.04$ dex, respectively. Cluster members show a significant spread in [Fe/H] and [Al/Fe] that is larger than expected based on measurement errors. An [Al/Fe] spread, signal of an Mg-Al anticorrelation is observed and used to constraint the cluster mass budget, along with C, N, Mg, Si, K, Ca, and Ce element variations are discussed. Across all the analysed evolutionary stages (RGB and AGB), about $\sim2/3$ (14 out of 23) show distinct chemical patterns, possibly associated with second-generation stars.

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arXiv2021

Unveiling short period binaries in the inner VVV bulge

Most of our knowledge about the structure of the Milky Way has come from the study of variable stars. Among the variables, mimicking the periodic variation of pulsating stars, are the eclipsing binaries. These stars are important in astrophysics because they allow us to directly measure radii and masses of the components, as well as the distance to the system, thus being useful in studies of Galactic structure alongside pulsating RR Lyrae and Cepheids. Using the distinguishing features of their light curves, one can identify them using a semi-automated process. In this work, we present a strategy to search for eclipsing variables in the inner VVV bulge across an area of 13.4 sq. deg. within $1.68^{\rm o}<l<7.53^{\rm o}$ and $-3.73^{\rm o}<b<-1.44^{\rm o}$, corresponding to the VVV tiles b293 to b296 and b307 to b310. We accurately classify 212 previously unknown eclipsing binaries, including six very reddened sources. The preliminary analysis suggests these eclipsing binaries are located in the most obscured regions of the foreground disk and bulge of the Galaxy. This search is therefore complementary to other variable stars searches carried out at optical wavelengths.

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arXiv2021

Infrared photometry and CaT spectroscopy of globular cluster M 28 (NGC 6626)

Recent studies show that the inner Galactic regions host genuine bulge globular clusters, but also halo intruders, complex remnants of primordial building blocks, and objects likely accreted during major merging events. In this study we focus on the properties of M 28, a very old and massive cluster currently located in the Galactic bulge. We analysed wide-field infrared photometry collected by the VVV survey, VVV proper motions, and intermediate-resolution spectra in the calcium triplet range for 113 targets in the cluster area. Our results in general confirm previous estimates of the cluster properties available in the literature. We find no evidence of differences in metallicity between cluster stars, setting an upper limit of Delta[Fe/H]<0.08 dex to any internal inhomogeneity. We confirm that M 28 is one of the oldest objects in the Galactic bulge (13-14 Gyr). From this result and the literature data, we find evidence of a weak age-metallicity relation among bulge globular clusters that suggests formation and chemical enrichment. In addition, wide-field density maps show that M 28 is tidally stressed and that it is losing mass into the general bulge field. Our study indicates that M 28 is a genuine bulge globular cluster, but its very old age and its mass loss suggest that this cluster could be the remnant of a larger structure, possibly a primeval bulge building block.

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arXiv2021

Discovery of new globular clusters in the Sagittarius dwarf galaxy

Context. Globular clusters (GCs) are witnesses of the past accretion events onto the Milky Way (MW). In particular, the GCs of the Sagittarius (Sgr) dwarf galaxy are important probes of an on-going merger. Aims. Our main goal is to search for new GC members of this dwarf galaxy using the VISTA Variables in the Via Lactea Extended Survey (VVVX) near-infrared database combined with the Gaia Early Data Release 3 (EDR3) optical database. Methods. We investigated all VVVX-enabled discoveries of GC candidates in a region covering about 180 sq. deg. toward the bulge and the Sgr dwarf galaxy. We used multiband point-spread function photometry to obtain deep color-magnitude diagrams (CMDs) and luminosity functions (LFs) for all GC candidates, complemented by accurate Gaia-EDR3 proper motions (PMs) to select Sgr members and variability information to select RR Lyrae which are potential GC members. Results. After applying a strict PM cut to discard foreground bulge and disk stars, the CMDs and LFs for some of the GC candidates exhibit well defined red giant branches and red clump giant star peaks. We selected the best Sgr GCs, estimating their distances, reddenings, and associated RR Lyrae. Conclusions. We discover 12 new Sgr GC members, more than doubling the number of GCs known in this dwarf galaxy. In addition, there are 11 other GC candidates identified that are uncertain, awaiting better data for confirmation.

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arXiv2021

Assessing the stellar population and the environment of an HII region on the far side of the Galaxy

We have investigated the stellar and interstellar content of the distant star formation region IRAS 17591-2228 (WISE HII region GAL 007.47+0.06). It is associated to a water maser, whose parallax distance is d=20.4^{+2.8} {-2.2} kpc, supported by independent measurements of proper motion and radial velocity. It is projected in the same direction as an extremely red (J-Ks ~ 6 mag) group of stars, and a shell of mid-infrared emission. We qualify the group of stars as a cluster candidate, VVV CL177. Its radius spans between 0.45' and 1' and contains at least two young stellar objects with an extreme extinction near Av ~ 40 mag. Yet more analysis will be required to determine is it is a real single cluster associated with the water maser. The 13CO emissions at the radial velocity of the maser corresponds to the mid-infrared emission.

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arXiv2021

Confirmation and physical characterization of the new bulge globular cluster Patchick 99 from the VVV and Gaia surveys

Globular clusters (GCs) are important tools to understand the formation and evolution of the Milky Way (MW). The known MW sample is still incomplete, so the discovery of new GC candidates and the confirmation of their nature are crucial for the census of the MW GC system. Our goal is to confirm the physical nature of two GC candidates: Patchick99 and TBJ3, located towards the Galactic bulge. We use public data in the near-IR from the VVV, VVVX and 2MASS along the with deep optical data from the Gaia DR2, in order to estimate their main physical parameters: reddening, extinction, distance, luminosity, mean cluster proper motions (PMs), size, metallicity and age. We investigate both candidates at different wavelengths. We use near-IR and optical CMDs in order to analyse Patchick99. We decontaminate CMDs following a statistical procedure and PM-selection. Reddening and extinction are derived by adopting reddening maps. Metallicity and age are evaluated by fitting stellar isochrones. Reddening and extinction are E(J-Ks)=0.12+/-0.02 mag, AKs=0.09+/-0.01 mag from the VVV data, whereas E(BP-RP)=0.21+/-0.03 mag, AG=0.68+/-0.08 mag from Gaia DR2. We estimate a distance d=6.4+/-0.2 kpc in near-IR and D=7.0+/-0.2 kpc in optical. We derive its metallicity and age fitting PARSEC isochrones, finding [Fe/H]=-0.2+/-0.2 dex and t=10+/-2 Gyr. The mean PMs for Patchick99 are pmRA=-298+/-1.74 mas/yr and pmDEC=-5.49+/-2.02 mas/yr. We confirm that it is a low-luminosity GC, with MKs=-7.0+/-0.6 mag. The radius estimation is performed building the radial density profile, finding r~10'. We recognise 7 RR Lyrae star members within 8.2 arcmin from its centre, confirming the distance found by other methods. We found that TBJ3 shows mid-IR emissions that are not present in GCs. We discard TBJ3 as GC candidate and we focus on Patchick99. We conclude that it is an old metal-rich GC, situated in the Galactic bulge.

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arXiv2021

Gemini/Phoenix H-band analysis of the globular cluster AL3

The globular cluster AL~3 is old and located in the inner bulge. Three individual stars were observed with the Phoenix spectrograph at the Gemini South telescope. The wavelength region contains prominent lines of CN, OH, and CO, allowing the derivation of C, N, and O abundances of cool stars. We aim to derive C, N, O abundances of three stars in the bulge globular cluster AL3, and additionally in stars of NGC 6558 and HP1. The spectra of AL3 allows us to derive the cluster's radial velocity. For AL3, we applied a new code to analyse its colour-magnitude diagram. Synthetic spectra were computed and compared to observed spectra for the three clusters. We present a detailed identification of lines in the spectral region centred at 15555 A, covering the wavelength range 15525-15590 A. C, N, and O abundances are tentatively derived for the sample stars.

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arXiv2020

A hundred new eclipsing binary system candidates studied in a near-infrared window in the VVV survey

We present the first results obtained from an extensive study of eclipsing binary (EB) system candidates recently detected in the VISTA Variables in the Vía Láctea (VVV) near-infrated (NIR) Survey. We analyze the VVV tile d040 in the southern part of the Galactic disc wherein the interstellar reddening is comparatively low, which makes it possible to detect hundreds of new eclipsing binary candidates. We present here the light curves and the determination of the geometric and physical parameters of the best candidates found in this "NIR window", including 37 contact, 50 detached and 13 semi-detached eclipsing binary systems. We infer that the studied systems have an average of the $K_s$ amplitudes of $0.8$ mag and a median period of 1.22 days where, in general, contact binaries have shorter periods. Using the "Physics Of Eclipsing Binaries" (PHOEBE) interactive interface, which is based on the Wilson and Devinney code, we find that the studied systems have low eccentricities. The studied EBs present mean values of about 5700 K and 4900 K for the $T_1$ and $T_2$ components, respectively. The mean mass-ratio ($q$) for the contact EB stars is $\sim$ 0.44. This new Galactic disc sample is a first approach to the massive study of NIR EB systems.

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arXiv2020

How Many Components? Quantifying the Complexity of the Metallicity Distribution in the Milky Way Bulge with APOGEE

We use data of $\sim$13,000 stars from the SDSS/APOGEE survey to study the shape of the bulge MDF within the region $|\ell|\leq11^\circ$ and $|b|\leq13^\circ$, and spatially constrained to ${\rm R_{GC}\leq3.5}$ kpc. We apply Gaussian Mixture Modeling and Non-negative Matrix Factorization decomposition techniques to identify the optimal number and the properties of MDF components. We find the shape and spatial variations of the MDF (at ${\rm [Fe/H]\geq-1}$ dex) are well represented as a smoothly varying contribution of three overlapping components located at [Fe/H]=+$0.32$, $-0.17$ and $-0.66$ dex. The bimodal MDF found in previous studies is in agreement with our trimodal assessment once the limitations in sample size and individual measurement errors are taken into account. The shape of the MDF and its correlations with kinematics reveal different spatial distributions and kinematical structure for the three components co-existing in the bulge region. We confirm the consensus physical interpretation of metal-rich stars as associated with the secularly evolved disk into a boxy/peanut X-shape bar. On the other hand, metal-intermediate stars could be the product of in-situ formation at high redshift in a gas-rich environment characterized by violent and fast star formation. This interpretation would help to link a present-day structure with those observed in formation in the center of high redshift galaxies. Finally, metal-poor stars may correspond to the metal-rich tail of the population sampled at lower metallicity from the study of RR Lyrae stars. Conversely, they could be associated with the metal-poor tail of the early thick disc.

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arXiv2020

Mapping the stellar age of the Milky Way bulge with the VVV. III. High resolution reddening map

The detailed study of the Galactic bulge stellar population necessarily requires an accurate representation of the interstellar extinction particularly toward the Galactic plane and center, where the severe and differential reddening is expected to vary on sub-arcmin scales. Although recent infrared surveys have addressed this problem by providing extinction maps across the whole Galactic bulge area, dereddened color-magnitude diagrams near the plane and center appear systematically undercorrected, suggesting the need for higher resolutions. These undercorrections affect any stellar study sensitive to color (e.g. star formation history analysis via color-magnitude diagram fitting), either making them inaccurate or limiting them to small low/stable extinction windows where this value is better constrained. We aim at providing a high-resolution (2 arcmin to $\sim$ 10 arcsec) color excess map for the VVV bulge area, in $\mathrm{J}-\mathrm{K}_s$ color. We use the MW-BULGE-PSFPHOT catalogs sampling $\sim$ 300 deg$^2$ across the Galactic bulge ($|l| < 10^\circ$ and $-10^\circ < b < 5^\circ$) to isolate a sample of red clump and red giant branch stars, for which we calculate average $\mathrm{J}-\mathrm{K}_s$ color in a fine spatial grid in $(l, b)$ space. We obtain a E$(\mathrm{J}-\mathrm{K}_s)$ map spanning the VVV bulge area of roughly 300 deg$^2$, with the equivalent to a resolution between $\sim$ 1 arcmin for bulge outskirts ($l < -6^\circ$) to below 20 arcsec within the central $|l| < 1^\circ$, and below 10 arcsec for the innermost area ($|l| < 1^\circ$ and $|b| < 3^\circ$). The result is publicly available at http://basti-iac.oa-teramo.inaf.it/vvvexmap/

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arXiv2020

VVVX-Gaia Discovery of a Low Luminosity Globular Cluster in the Milky Way Disk

Milky Way globular clusters (MW GCs) are difficult to identify at low Galactic latitudes because of high differential extinction and heavy star crowding. The new deep near-IR images and photometry from the VISTA Variables in the Via Láctea Extended Survey (VVVX) allow us to chart previously unexplored regions. Our long term aim is to complete the census of MW GCs. The immediate goals are to estimate the astrophysical parameters, measuring their reddenings, extinctions, distances, total luminosities, proper motions, sizes, metallicities and ages. We use the near-IR VVVX survey database, in combination with Gaia DR2 optical photometry, and with the Two Micron All Sky Survey (2MASS) photometry. We report the detection of a heretofore unknown Galactic Globular Cluster at $RA =$ 14:09:00.0; $DEC=-$65:37:12 (J2000). We calculate a reddening of $E(J-K_s)=(0.3\pm 0.03)$ mag and an extinction of $A_{K_s}=(0.15\pm 0.01)$ mag for this new GC. Its distance modulus and corresponding distance were measured as $(m-M)=(15.93\pm0.03)$ mag and $D=(15.5\pm1.0)$ kpc, respectively. We estimate the metallicity and age by comparison with known GCs and by fitting PARSEC and Dartmouth isochrones, finding $[Fe/H]=(-0.70\pm0.2)$ dex and $t=(11.0\pm1.0)$ Gyr. The mean GC PMs from Gaia are $μ_{α^\ast}=(-4.68 \pm 0.47 )$ mas $yr^{-1}$ and $μ_δ=(-1.34 \pm 0.45)$ mas $yr^{-1}$. The total luminosity of our cluster is estimated to be $M_{Ks}=(-7.76\pm 0.5)$ mag. We have found a new low-luminosity, old and metal-rich globular cluster, situated in the far side of the Galactic disk, at $R_{G}=11.2$ kpc from the Galactic centre, and at $z=1.0$ kpc below the plane. Interestingly, the location, metallicity and age of this globular cluster are coincident with the Monoceros Ring (MRi) structure.

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arXiv2020

Small scale star formation as revealed by VVVX galactic cluster candidates

We report a search and analysis of obscured cluster candidates in the "VISTA Variables in the Via Lactea eXtended (VVVX)" ESO Public Survey area encompassing the region between 229.4 < l < 295.2 and -4.3 < b < 4.4 of the southern Galactic disk. We discover and propose 88 new clusters. We improve the completeness of the embedded cluster population in this region, adding small size (linear diameters of 0.2-1.4 pc) and relatively far objects (heliocentric distance between 2 and 4 kpc) to existing catalogues. Nine candidates are proposed to be older open cluster candidates. Three of them (VVVX CL204, 207, 208) have sufficient numbers of well-resolved stellar members to allow us to determine some basic cluster parameters. We confirm their nature as older, low-mass open clusters. Photometric analysis of 15 known clusters shows that they have ages above 20Myr, and masses below 2000Msun: in general, their proper motions follow the motion of the disk. We outline some groups of clusters, most probably formed within the same dust complex. Broadly, our candidates follow the network of filamentary structure in the remaining dust. Thus, in this part of the southern disk of the Galaxy, we have found recent star formation, producing small size and young clusters, in addition to the well known, massive young clusters, including NGC3603, Westerlund2 and the Carina Nebula Complex.

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arXiv2020

Discovery of a mid-infrared protostellar outburst of exceptional amplitude

We report the discovery of a mid-infrared outburst in a Young Stellar Object (YSO) with an amplitude close to 8 mag at $λ$$\approx$4.6 $μ$m. WISEA J142238.82-611553.7 is one of 23 highly variable WISE sources discovered in a search of Infrared Dark Clouds (IRDCs). It lies within the small IRDC G313.671-0.309 (d$\approx$2.6 kpc), seen by the Herschel/HiGal survey as a compact, massive cloud core that may have been measurably warmed by the event. Pre-outburst data from Spitzer in 2004 suggest that it is a class I YSO, a view supported by observation of weak 2.12 $μ$m H$_2$ emission in an otherwise featureless red continuum spectrum taken in 2019 (6 mag below the peak in K$_s$). Spitzer, WISE and VVV data indicate that the outburst began by 2006 and has a duration $>$13 yr, with a fairly flat peak from 2010--2014. The outburst luminosity of a few $\times 10^2$ Lsun is consistent with an accretion rate Mdot $\approx 10^{-4}$ Msun/yr, comparable to a classical FU Orionis event. The 4.6 $μ$m peak in 2010 implies T = 800-1000 K and a disc radial location R$\approx$4.5 au for the emitting region. The colour evolution suggests subsequent progression outward. The apparent absence of the hotter matter expected in thermal instability or MRI models may be due to complete obscuration of the innermost disc, e.g. by an edge-on disc view. Alternatively, disc fragmentation/infalling fragment models might more naturally explain a mid-infrared peak, though this is not yet clear.

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arXiv2020

Cool stars in the Galactic Center as seen by APOGEE: M giants, AGB stars and supergiant stars/candidates

The Galactic Center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool AGB stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the RGB.We study metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE/ASPCAP pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic Center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ~ +0.3 dex and a metal-poor peak around [Fe/H]= -0.5 dex, which is 0.2 dex poorer than Baade's Window. The alpha-elements Mg, Si, Ca, and O show a similar trend to the Galactic Bulge. The metal-poor component is enhanced in the alpha-elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high velocity stars with $\rm v_{gal} > 300\,km/s$; the metal-rich stars show a much higher rotation velocity ($\rm \sim 200\,km/s$) with respect to the metal-poor stars ($\rm \sim 140\,km/s$). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disc and the nuclear star cluster show distinct chemical signatures and might be formed differently.

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arXiv2020

The VISCACHA survey -- II. Structure of star clusters in the Magellanic Clouds periphery

We provide a homogeneous set of structural parameters of 83 star clusters located at the periphery of the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). The clusters' stellar density and surface brightness profiles were built from deep, AO assisted optical images, and uniform analysis techniques. The structural parameters were obtained from King and Elson et al. model fittings. Integrated magnitudes and masses (for a subsample) are also provided. The sample contains mostly low surface brightness clusters with distances between 4.5 and 6.5 kpc and between 1 and 6.5 kpc from the LMC and SMC centres, respectively. We analysed their spatial distribution and structural properties, comparing them with those of inner clusters. Half-light and Jacobi radii were estimated, allowing an evaluation of the Roche volume tidal filling. We found that: (i) for our sample of LMC clusters, the tidal radii are, on average, larger than those of inner clusters from previous studies; (ii) the core radii dispersion tends to be greater for LMC clusters located towards the southwest, with position angles of $\sim$200 degrees and about $\sim$5 degrees from the LMC centre, i.e., those LMC clusters nearer to the SMC; (iii) the core radius evolution for clusters with known age is similar to that of inner clusters; (iv) SMC clusters with galactocentric distances closer than 4 kpc are overfilling; (v) the recent Clouds collision did not leave marks on the LMC clusters' structure that our analysis could reveal.

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arXiv2020

Optical spectroscopic classification of 35 hard X-ray sources from the Swift-BAT 70-month catalogue

The nature of a substantial percentage (about one fifth) of hard X-ray sources discovered with the BAT instrument onboard the Neil Gehrels Swift Observatory (hereafter Swift) is unknown because of the lack of an identified longer-wavelength counterpart. Without such follow-up, an X-ray catalogue is of limited astrophysical value: we therefore embarked, since 2009, on a long-term project to uncover the optical properties of sources identified by Swift by using a large suite of ground-based telescopes and instruments. In this work, we continue our programme of characterization of unidentified or poorly studied hard X-ray sources by presenting the results of an optical spectroscopic campaign aimed at pinpointing and classifying the optical counterparts of 35 hard X-ray sources taken from the 70-month BAT catalogue. (...) With the use of optical spectra taken at six different telescopes we were able to identify the main spectral characteristics (continuum type, redshift, and emission or absorption lines) of the observed objects, and determined their nature. We identify and characterize a total of 41 optical candidate counterparts corresponding to 35 hard X-ray sources given that, because of positional uncertainties, multiple lower energy counterparts can sometimes be associated with higher energy detections. We discuss which ones are the actual (or at least most likely) counterparts based on our observational results. In particular, 31 sources in our sample are active galactic nuclei: 16 are classified as Type 1 (with broad and narrow emission lines) and 13 are classified as Type 2 (with narrow emission lines only); two more are BL Lac-type objects. We also identify one LINER, one starburst, and 3 elliptical galaxies. The remaining 5 objects are galactic sources: we identify 4 of them as cataclysmic variables, whereas one is a low mass X-ray binary.

astro-ph.HE↗

arXiv2020

Using classical Cepheids to study the far side of the Milky Way disk: I. Spectroscopic classification and the metallicity gradient

The structure, kinematics, and chemical composition of the far side of the Milky Way disk, beyond the bulge, are still to be revealed. Classical Cepheids (CCs) are young and luminous standard candles. We aim to use a well-characterized sample of these variable stars to study the present time properties of the far side of the Galactic disk. A sample of 45 Cepheid variable star candidates were selected from near infrared time series photometry obtained by the VVV survey. We characterized this sample using high quality near infrared spectra obtained with VLT/X-Shooter, deriving radial velocities and iron abundances for all the sample Cepheids. This allowed us to separate the CCs, which are metal rich and with kinematics consistent with the disk rotation, from type II Cepheids (T2Cs), which are more metal poor and with different kinematics. We estimated individual distances and extinctions using VVV photometry and period-luminosity relations, reporting the characterization of 30 CCs located on the far side of the Galactic disk, plus 8 T2Cs mainly located in the bulge region, of which 10 CCs and 4 T2Cs are new discoveries. This is the first sizeable sample of CCs in this distant region of our Galaxy that has been spectroscopically confirmed. We use their positions, kinematics, and metallicities to confirm that the general properties of the far disk are similar to those of the well-studied disk on the solar side of the Galaxy. In addition, we derive for the first time the radial metallicity gradient on the disk's far side. Considering all the CCs with $R_{\mathrm{GC}} < 17\,\rm{kpc}$, we measure a gradient with a slope of $-0.062 \, \mathrm{dex\, kpc^{-1}}$ and an intercept of $+0.59 \, \rm{dex}$, which is in agreement with previous determinations based on CCs on the near side of the disk.

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